Deism is the name given to any worldview or religion that insists that a god exists, yet has nothing to do with the present. An analogy that is often offered is that god is like a watchmaker who built the watch, wound it up, and walked away.
Since this god is not involved in the present, it follows that he did not create the present. If he did not create the present, he is not in charge of time. If he is not in charge of time, time does not have to behave the way he wants it to.
Humans begin life believing that all things, including time generally, and the present specifically, are without contradiction. Nothing is and is-not at the same time and in the same way. Including time.
But what is the basis for this belief? Is it justified rationally? Or is it an irrational assumption?
If it is an irrational assumption, humans cannot claim that any conclusion is rationally justified, since absolutely every conclusion is based on the assumption of non-contradiction.
In other words, if non-contradiction is merely an irrational assumption, it would be no more reasonable to say that 1 + 1 = 2 than to say that 1 + 1 = 5,987,162. Or than to say that 1 + 1 = yellow. Or anything else for that matter.
The only other option is that non-contradiction is, in fact, a rationally justified belief.
However, since it's a starting belief, a first-principle, an initial presupposition, it is not a conclusion that we draw. Instead, it is believed by us before every conclusion we draw.
How can we rationally justify this belief then, if it is not the result of our own rational thought processes?
The answer is that we cannot.
But a rational God can.
Only a rational God can rationally justify our initial beliefs. If our initial beliefs are the product of random accidents, as atheists claim, then these beliefs begin to exist without reason, making them irrational.
While deism allows for god to have caused our initial beliefs, deism does not allow for god to actually know what he is talking about. The deistic god, by definition, did not create time. Time does not have to behave the way he wants it to. How would he know if time is non-contradictory? He may very well have caused us to believe that it is, but he himself has no reason to believe that it is! This makes our initial belief of non-contradiction a belief held without rational justification -- an irrational belief!
In the end, deism is nothing more than atheism repackaged. There is no God in diesm who can actually justify human thought.
This makes deism deductively false.