As a ‘friendly atheist,’ I believe that theists can be fully rational in believing that God exists. I know that some theists are ‘unfriendly theists’ in the sense that they believe
(1) The existence of God is obvious to everyone.
(2) There is a supernatural creator of the universe
(~1) God’s existence is not obvious to everyone.
One could be persuaded that God exists on the basis of arguments like the kalam cosmological argument, the fine-tuning argument, etc., without believing that God’s existence is obvious to everyone. So is there any non-question-begging argument for (1)?
Here is one such argument:
(3) If X is a causal condition of Y’s rationality, then in order to be rational Y must presuppose X.
(4) God is a causal condition of morality, logic, etc.
(5) Therefore, insofar as we are rational, we must presuppose God.
For the sake of argument, let’s grant the truth of the highly doubtful (4). Even so, (3) is false. From the fact that X is a causal condition of Y’s rationality, it doesn’t follow that, in order to be rational, Y must presuppose X.
Consider the following analogy. Human life “presupposes” the existence of carbon atoms. If there were no carbon atoms, there would not be any human life. Yet there are many people in the world who are completely ignorant of carbon atoms or atoms in general, for they have little to no understanding of science. Therefore, although human life “presupposes” the existence of carbon atoms, there are many people who do not have an awareness of that fact. Similarly, from the fact, if it is a fact, that morality, logic, etc. “presuppose” God’s existence, it doesn’t follow that all human beings have an awareness of God’s existence. Additional premises would be needed to show how a ‘presuppositional relationship’ entails awareness of that relationship.
Autobiographical aside: it seems to me that (1) is not only false, but obviously false. Indeed, I find it interesting that some really smart people could honestly believe (1), despite the fact that I think it is obviously false. Of course, those same people could reply that I’m self-deceived.