In the words of the late Greg Bahnsen, the most basic proof of Christianity is that if it were not true, it would be impossible to prove anything at all.
If the non-Christian worldview is true, the laws of logic are not valid. Therefore, even though non-Christians use logic to interpret evidence, they have no real justification for doing so. In the non-Christian worldview, logic is not something that can be trusted.
Christians alone do have a basis for recognizing that the law of non-contradiction applies everywhere and at every time, past, present and future. Our basis is the character of our God.
According to the Bible, the Creator of all things is consistently logical. He cannot lie (Hebrews 6:18, Titus 1:2) and He cannot deny Himself (2 Timothy 2:13).
Further, all truth and knowledge is in Christ (Colossians 2:3), who is God (John 1:1), and who created all things (John 1:3).
As Christians we know that all things have been created by One who is consistently logical, and therefore the laws of logic are both universal and invariant. Because He holds the keys to all knowledge, we also know that logical knowledge is possible.
Christianity, as we see, has grounds for trusting logic -- for claiming that the laws of logic are always true, a claim that other worldviews have no basis for making.
An easy way to demonstrate this latter point, that non-Christian worldviews deny the validity of the laws of logic, is to begin at the beginning.
We see that time past has ended. To avoid infinite regress, we must acknowledge that it also began.
Infinite regress is logically incoherent. To suggest that there have been an infinite regress of past moments is to suggest that we have come to the end of an infinite series. An infinite series, however, is by definition a series with no end. So this would mean that we have come to the end of a series with no end, which is logically incoherent.
Infinite regress would mean that we have iterated, one-by-one, through every single item of an infinite series. But an infinite series always has more items than those that have been iterated through. We would have iterated through something that cannot be iterated through, which is logically incoherent.
If we were to go backwards through each previous moment in past time, and there were an infinite number of past moments, there would necessarily be some moment in the set of previous moments that we would never, ever get to. If that were not the case, it would not be infinite. If there is some supposed prior moment in the set of previous moments that we could never get to while iterating backwards through all previous moments, then, iterating forward from that moment to the present moment, we would never arrive at the present moment for the same reason that going backwards we could never arrive at the previous moment -- namely, the infinite (or unending) number of moments in between the two.
Infinite regress would mean that we have completed something that cannot be completed. We have traversed something that cannot be traversed. We have itemized what cannot be itemized, counted what cannot be counted, spanned what cannot be spanned.
Infinite regress violates the logical law of non-contradiction.
So we quickly see that Time Past is not infinite, and must therefore have been finite.
THE BEGINNING WAS CAUSED
To suggest that time began uncaused, or anything beginning uncaused for that matter, is absurd... it is the same as suggesting that having nothing and adding nothing to it, something results, or in other words:
0 + 0 > 0
I like to refer to this kind of thinking as "Atheist Math". In Atheist Math, zero is not equal to zero, violating the logical law of Identity.
Ultimately, there is some kind of cause of time, and to again avoid infinite regress, there must be an ultimate First Cause of time.
THE CAUSE IS ETERNAL
If the cause of time were somehow temporal, it would require the passing of time to exist, making what's known as a circular dependency, another type of infinite regress, which violates the logical law of non-contradiction.
Therefore, being not bound by time, it is, by definition, eternal (Deuteronomy 33:27).
Since it is outside of time, it does not inherently change over time, which means its inherent qualities are unchanging (Malachi 3:6).
THE CAUSE IS PERSONAL
Since it is the First Cause, it must be uncaused.
Since it is the uncaused first cause, it is self-directed and self-motivated and acted volitionally, which, simply by definition, makes it personal (Job 13:8). If it were not personal, it would not be capable of self-directed, self-motivated, volitional action, preventing it from being the First Cause. It would be the First Cause and not be the First Cause, violating the logical law of non-contradiction.
ATHEISM THUS INCOHERENT
Thus, a personal, unchanging and eternal uncaused First Cause clearly exists because of the logical impossibility of the contrary. This rules out all possibility of Atheism, Agnosticism, Ignosticism, and any worldview that claims that past time has been infinite, or that the Divine Initiator is temporal.
Every temporal action or activity has to have a personal, uncaused First Cause ultimately behind it, from the movement of light away from the sun, to the decision to share your last chocolaty caramel Rollo with the person you love, to our very recognition of right and wrong. Any event that takes time had to begin, since time itself began, and if that event began, then it must have been caused, and if it was caused, then there must ultimately be a personal uncaused First Cause behind it.
This tells us that if there are multiple personal uncaused First Causes, multiple ultimate authorities, they must be working in perfect eternal unity. Otherwise, in areas in which they disagreed, things would be and not be, or happen and not happen, at the same time and in the same way. That would be logically incoherent, again violating the logical law of non-contradiction, thereby rendering knowledge of any kind absolutely impossible.
So there must be either one singular ultimate authority (known as Unitarian Monotheism), or else all these personal authorities must be in perfect, eternal and divine coordination with each other, of one mind, one purpose, in total unity, and as such a truly singular authority, like the three persons of the Christian Trinity (Trinitarian Monotheism), for example.
WE DON'T DO WHAT WE OUGHT
We are each keenly aware that this unified personal ultimate authority, or "God", has caused us to have a guiding sense of moral justice which we call our conscience, and we are also aware that we are unable to perfectly obey it.
Man's disobedience to the ultimate authority serves as a foundational problem resulting in the establishment of essentially every religion.
However, non-Christian religions tell us that the solution is that we must do better -- that we must be perfect -- that we must fix the problem of sin.
WE ARE IN DEBT
God has provided us with time in this life during which we ought to obey the moral code He gave us. If any amount of time is spent in disobedience, we have spent that time in a way other than we should have. Since the time was provided to us by the God that also informed us of how we should use it, and since we have not used it all as we ought, to right this wrong, additional time must now be spent the way the wasted time ought to have been spent.
Among other things, we owe God time.
WE CAN'T FIX THE PROBLEM
We cannot create additional time for ourselves. Any time we have was provided to us by God to be used in obedience to Him. Any more time that God gives us will only increase our debt to Him, and, since the ultimate authority is absolutely united, there is no other way for us to get extra time.
Suggesting that we can pay back our debt of time in submission to our Creator "on our own" is really no different than suggesting that we should borrow from our lender to pay him back for money we borrowed from him earlier. The debt would never get paid that way.
ALL REMAINING WORLDVIEWS INCOHERENT
To claim that we can pay off a debt to our Creator is to claim that we own something that He has no authority over. But we have already established, via the necessary perfect unity of all Personal First Causes, that absolutely everything we own comes from Him, and that He has authority over all of it.
Thus, to suggest we can pay Him back for what we have misused is to suggest that the One in authority over everything is not in authority over everything. This again violates the logical law of non-contradiction, disqualifying all remaining worldviews.
Christianity, by contrast, tells us the only possible truth -- that we aren't in charge, and we can't be perfect: we can't fix the problem of sin.
GOD CAN FIX THE PROBLEM
However, self-evidently, the unified One in ultimate authority can fix the problem. (Mark 10:26-27)
In fact, He has.
Only Christianity tells us about the God who loves us so much that He took our penalty on His shoulders -- it tells us that, in addition to the Son's eternal and perfect obedience to the Father in our place, He also shed His own blood on the cross to pay the debt of our sin, provided we accept His free gift (Romans 6:23; John 3:16; Acts 20:28; Revelation 1:5; Romans 10:9).
Only Christianity fully recognizes the reality of sin, the inability of man to fix the problem, and the graciousness of a perfect God to solve it for us.
Ultimately, all the clear and unquestionable proofs in the world cannot and will not convince a rebellious person to trust his Creator, because he intentionally suppresses the truth in unrighteousness (Romans 1:18).
Some claim that God's existence cannot be proven, since, if we could prove His existence, we would not need to have faith. However, this is simply not the case. God tells us that everyone knows He exists (Romans 1:20). Trusting the God that we all know exists is what the Bible calls "faith". Without this faith, it is impossible to please God (Hebrews 11:6).
So we see that the Christian has every justification to assert his worldview as absolute truth, while the non-Christian, were his views true, would be incapable of asserting anything at all. If Christianity were not true, the laws of logic would be invalid, and every assertion would be meaningless.
The most basic proof of Christianity is that if it were not true, it would be impossible to prove anything at all.
How Can We Know the Bible is Valid?
Naturalism is currently the most scientific form of atheism. Naturalists believe that given what we know at present, Naturalism is probably true. Our evidence comes from all the sciences, but also from what we have not discovered despite constant searching: applying the best methods and the most disciplined standards, we have failed to find convincing evidence of any supernatural power or being. Evidence presented for such things has always turned out on close examination to be fabricated, misunderstood, or misrepresented.
In contrast, we have consistently found natural causes and explanations for the things we observe and experience, for centuries now, and the results have steadily led us toward a unifying, naturalistic worldview, never away from it. This makes it highly probable that everything else will turn out to have a natural explanation, too. Because so far, everything we have thoroughly investigated has done so, without exception. We simply have never found anything else to exist, or to have any other cause, than matter-energy and space-time. So probably that is all that does exist. This is the first and most important reason we believe naturalism is probably true.
We have countless examples of how the evidence has panned out this way. The cause of lightning was once thought to be God's wrath, but now we know it is the unintelligent outcome of mindless natural forces. The solar system was once thought to be incredible proof of an intelligent being arranging and maintaining the orderly motions, but now we know it is all the inevitable and predictable outcome of mindless natural forces. Disease was once thought to be the mischief of demons, but now we know that tiny unintelligent organisms are the cause, which reproduce and infect us according to mindless natural forces.
Likewise, the amazing organization of animal and plant bodies was once thought to be proof of instantaneous creation by God, but now we have an enormous and growing body of evidence that this is all the product of an amazingly long and blindly meandering process of natural selection, which is in turn the inevitable outcome of mindless natural forces. The story has been the same, again and again, for everything we thoroughly examine. When we see that the same horse keeps winning, millions of times over, and no other horse has ever even crossed the finish line, of course we decide to bet on the winning horse.
But even where we have yet to gather all the required evidence to be sure, the evidence we do have continues to point in this same direction. This is clear even on the three greatest frontiers of scientific inquiry: the origin of the universe, the origin of life, and the nature of consciousness. Science is still collecting the evidence it needs to answer these questions, so the scientific verdict is not yet in. But the verdict of philosophical probability is already clear: not only from all past precedent (like the story of lightning, the solar system, disease, evolution, etc.), but from what scientists have already found out about the universe, life, and the human mind. There is far more to each story than can be covered here, and though we believe it all points to Naturalism, we shall only look at some examples below.
The second most important reason we believe Naturalism is that it predicts the universe must be a certain way, and it just so happens that the universe turns out to be exactly the way it has to be if Naturalism is true, yet not at all the way we would expect on some other theory.
For example, according to Naturalism, life could only be an accidental byproduct of the organization of the universe, and the only way it could be that is if the universe is so incredibly old and so incredibly big that something as improbable as the origin of life would be possible. But we can think of no other good reason for the universe to be so old and big. A god, for example, has no good reason to make a universe that is over a hundred thousand times as old as the entire human race has even existed, and over a hundred billion times as large as the entire solar system. But on Naturalism, the universe has to be that old and large. Therefore, Naturalism predicts exactly the universe we find ourselves in, but Creationism does not, and in fact makes little sense of it.
We see this again and again, even in some obvious ways. The prevalence of pointless misery and harm caused by nature, and the utter indifference of the natural world toward human happiness, and the complete absence of any supernatural agency enforcing moral order or righting wrongs or fixing problems, makes no sense on any theory that a caring God exists, for example, but is the only way things could be if Naturalism is true. Once again, the evidence fits Naturalism.
Likewise, the total confusion and disagreement in the world as to any supernatural reality is also something Naturalism predicts, but that makes little sense if there is a supernatural reality. If there is none, then obviously no two cultures are going to "discover" the same reality, but each will invent its own opinions and conjectures completely different from every other. But if there was any genuine supernatural reality, everyone should be able to see the same thing.
We also observe that there is no way to resolve religious disagreement by appealing to the facts (as we can do in science) because it appears there are no religious facts to appeal to. So everyone who wants to promote his own religious "facts" can do so, and that is exactly what we observe: while science constantly unifies and achieves worldwide agreement across all cultures, religion constantly divides and diversifies, achieving no agreement at all, not even within a single culture. That makes little sense to us if there is any supernatural truth, but is exactly what we would expect if Naturalism is true.
Even on the three frontiers we see the same outcome:
In cosmology, for example, the leading scientific theories of the origin of our universe that are being investigated, which actually predict and explain the strange features and properties of the world (like why there are such strange things as "bottom quarks" or "neutrinos" or "Bose-Einstein condensates" or why the speed of light is what it is, and so on), involve no supernatural or intelligent beings, just natural causes and events, and yet they predict exactly what we observe: a universe that is almost entirely lethal to life (by far most of existence is a radiation-filled vacuum) and instead perfectly suited to black holes (which thrive in a radiation-filled vacuum, and to which most material in the universe is devoted to building). Indeed, we ask, why would our universe manufacture a trillion times more black holes than life-bearing planets? No other worldview makes much sense of this, and certainly none actually predict this strange observation--except, so far, Naturalism.
Likewise, when it comes to the origin of life, no other worldview makes much sense of the fact that life began as a single-celled organism constructed from a very simple self-replicating chemical (just four amino acids, with a simple chemical backbone), then took three billion years to finally stumble upon a multi-celled form, and then took a hundred million years after that to diversify into a set of standard optimal forms, and then took hundreds of millions years beyond that to develop even something as simple as a flower. Naturalism predicts exactly this, for this is the only pattern of development that is possible if Naturalism is true. But no other worldview readily predicts such a pattern.
The theories of life's origin that are now being investigated by scientists, again, involve no supernatural or intelligent beings, but appeal to nothing more than natural physics and chemistry. Yet they predict exactly what we observe: that life began as one tiny cell and only later evolved more complex forms, and only after an incredibly long time of trial and error, and that all life on earth is related and can be shown to have derived from some common ancestor that has since gone extinct (we have proven this the same way we prove the paternity of human fathers, but also from converging evidence in the fossil record). No other theory predicts this, nor does any other theory make much sense of it. Why, for example, do we need to be made of cells at all, each cell with a complete genome in it? We believe Naturalism explains this better than any other worldview.
Then there is the human mind. There is a lot about this that we still don't fully understand, but scientists are learning more every day, and what they have found so far points to Naturalism. We have identified how memories are stored, where emotions and reason operate or sensory experience is processed, and so on, and always we find that a physical part of the brain is required for each thing, so that if we take away each part, we take away the memories and abilities it contained, which means if we take away all the parts, we take away all that we are--our whole mind. That is what Naturalism predicts must be the case, but this is not what we'd expect if any other worldview was true.
The only way a mind could exist if Naturalism is true is if there is no other way to have a mind (and all the benefits of intelligence and consciousness) without a relatively gigantic machine to generate it, like our huge brain. But if Naturalism is not true, we could have consciousness without a huge, complex brain--because we could have something like souls, for example, which can't be injured, can't become damaged by wounds or disease, and don't hog the lion's share of our oxygen and food.
For instance, if God exists then obviously a mind can exist and function without a brain, so we wouldn't need brains any more than God does. So why do we have them? Naturalism alone predicts that we must. No other worldview makes as much sense of this. We believe our large, delicate brains are not what a God would give us, since they are hugely inefficient, consuming an enormous portion of our oxygen and food, they are easily damaged or harmed, they frequently malfunction, and they have to be so large that without modern medicine, nutrition, and hygiene, nearly one in seven mothers dies in childbirth. As far as we've seen, only Naturalism has a good explanation of why it has to be this way.
In every other case we look at, in any field of study, we find the same conclusion: once we actually investigate, we find that all the evidence supports Naturalism more than any other worldview. That is why we believe Naturalism is probably true. For more information see:
Naturalism as a Worldview
In our tradition pramana (evidence) is divided into several categories. The first and most important is revealed truth which comes to us in the form of sastra pramana (evidence from the revealed scriptures). Two other categories of evidence which are important as support to the first are called pratyaksha (experiential knowledge) and anumana (logical inference).
Because God is beyond the ability of the mind and intellect and is also not directly percieved by the senses our only hope of knowing him is if he chooses to reveal himself to us.
One can look at historical evidence, forensic evidence and other types of empirical evidence to try to verify claims made in sastra. This is a good exercise and will yield some helpful knowledge, but it will not prove all that is said in sastra.
Also, after having read the sastra and contemplated it's meaning a person can reflect on it and their own life experience and make some judgement as to the legitimacy of what is said in sastra. But that also will not prove everything that is said in sastra. It will convince most honest persons that there is some truth in sastra, but they will not be convinced by such an exercise that everything in sastra is true. Nor will the mere conviction that it is true actually directly yield spiritual vision.
Krsna gives an answer to Arjuna in Bhagavad-gita which is the perfect answer to this question. Arjuna has just been shown the universal form of God and asked to see him again in his original two armed form. Krsna explains to Arjuna that he cannot be seen (understand here proven) by study of the sastra, by penance, by austerity nor by charity. He says he can only seen seen as he is standing before Arjuna by those whose eyes are smeared with the salve of love. Only unto those who have uninterupted and unmotivated love and service to him does he reveal himself.
That brings up an intersting question. How can a person devote themselves 100% to loving service to Krsna is they don't know him? The answer is that a person can get a glimpse of who Krsna is from sastra and from their own intuition and life experience. That glimpse can grow by taking up practices recommended in sastra. As a persons vision becomes clearer they become more and more dedicated and faithful, eventually they will become a fully surrendered devotee. The process is gradual and depends on the sincerity and inner necessity of the practicioner.
Here is a mundane example to illustrate what I am talking about. Let's say someone tells you that they have buried a million dollars. They tell you the exact coordinates of the burial site and how deep the treasure if buried. They, for whatever reason, have no use for the treasure and you are free to uncover it. From such a description you will not know that there is indeed a treasure there. Based on the credibility of the individual (according to your judgement) and your necessity for the treasure you may decide to investigate. If you follow the persons instructions you will eventually know. But first you must have some faith or some necessity that drives you to action. Without that the treasure will be just an idea. In the same way sastra tells us all of the greatest treasure and how to uncover it. According to our judgement of the credibility of sastra and our inner necessity for such a divine treasure, we will act according to the instructions of sastra to uncover our treasure or that treasure will forever remain a mental construct with no verified proof of it's existence.
The proof of the pudding is in the tasting and likewise, the proof that Krsna is God and that you have a unique relationship with him can only be had by devotional service to him, by following the dictates of sastra.
In this age the process Krsna has recommended for finding the proof in your own conscious experience is the chanting of his holy names. By recitation of the holy name of God with sincerity and earnest yearning one will gain some inner experience and will find all the proof they need.