We know this because the contrary is impossible...
We know that God necessarily exists because of the impossibility of the contrary. We also know that God is good and that He cannot deny Himself for the same reason.
We know that we ought to behave in a certain way, and we know that we became aware of these oughts as a result of God's personal choice. In other words, He informed us of them. We know that we do not always obey the oughts -- we don't always do as He has told us to -- making us by definition opposed to what is right and true.
We also know that we cannot fix this problem. But, it is self-evident that, if God desires to fix it, He alone can.
Only Christianity places the solution entirely in the hands of God instead of placing it in the impotent hands of men (Ephesians 2:8-9).
Only Christianity reflects reality, and therefore, among opposing worldviews, only Christianity can possibly be true. If Christianity is true, then Christians do not worship a satan.
If you have never placed your trust in the God who made you, I invite you to do so. The Bible teaches that none of us have an excuse for our evil deeds, and none of us can justify our rebellion against our sovereign God (Romans 1:20).
But it also teaches that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved (Romans 10:9). You can be right with God, but only on His terms, not on your own.
Since Muslims more often than not suppress freedoms, predominately define their character by violent rage, are intolerant of nonbelievers, and defend all this as righteous, it would seem that they are indeed worshiping an evil god. Muslims who reject those wicked values rarely fight or speak up against other Muslims who endorse them, and thus like Nazi sympathizers in WWII, are complicit in the evil those other Muslims serve. That leaves a very small minority of Muslims who can claim to actually be worshiping and serving a good god, and yet they have no arguments to make that theirs is the true god, and not the evil god most other Muslims manifestly worship.
Christians likewise predominately worship and serve what they themselves define as the Antichrist, yet claim the Antichrist is in fact Christ, and Christ the Antichrist. If "Christ" is the spirit exemplified by the character of Jesus in the Gospels, serving Christ means despising all war, violence and wealth, giving all you have away to the poor, renouncing all divorce, and praying only in private (Jesus condemned public prayer as hypocrisy and commanded those who follow him to pray in a closet alone). Yet most Christians fiercely oppose socialist benefits for the poor, defend wealth and the most ruthless displays of capitalism, never give all their wealth away, support war and capital punishment and torture and killing in self-defense and the ownership of weapons, divorce and remarry as much as anyone, and insist on government-enforced public prayer.
If serving the "Antichrist" (which literally means "The Opposite of Christ") means serving exactly the opposite spirit to Christ, most Christians are manifestly serving the Antichrist. That they have been fooled into thinking otherwise is indeed actual proof that if there is a God, he is evil, and has fooled Christians into thinking the vile God they serve is good. Their actual God supports the accumulation of wealth, war, violence, torture, weapons, divorce, hypocritical displays of worship, and selfish disdain for the poor. That sure sounds like an evil god. Like the Muslims, Christians who reject those wicked values rarely fight or speak up against other Christians who endorse them, and thus like Nazi sympathizers in WWII, are complicit in the evil those other Christians serve. That leaves a very small minority of Christians who can claim to actually be worshiping and serving a good god, and yet they have no arguments to make that theirs is the true god, rather than the evil god most other Christians manifestly worship.
Of course, there are no gods, neither evil nor good, just the imaginary gods that people invent. The Antichrist is not a literal spirit, but an idea, like the spirit of the law or the spirit of democracy, which only becomes real insofar as people embody it and enact it. But that so many Muslims and Christians (apparently the majority of them) have fooled themselves into thinking that what they manifestly represent as an evil god, is in fact the paradigm of good, is frightening indeed, and is the cause of many of the world's ills.