That's a common question many proselytizing Evangelicals pose to atheists. If their mark answers that nothing would persuade them to believe in God, then the mark is accused of being close-minded, fanatical, and so on. If the mark gives answers that if he were to be persuaded that X, Y, & Z were shown to be true, then he would believe in God. Then the mark is asked, how can he be so sure that X, Y, & Z are not true? Further, in posing the question to the mark, the mark, even if only for a brief moment, imagines himself as someone who believes in God.
An essay on Ebon Musings with the title The Theist's Guide to Converting Atheists:
What would convince an atheist that a religion is true? reminded me of that stratagem.
I haven't finished working my way through the essay, but my initial reaction was something along the lines of "what would it take to show me that pigs can fly without using cannons?"
After I thought about the question for a while, I realized that my lack of belief in God stems not from believing certain things, but from aesthetics. My disbelief is not tied to the truth or falsity of specific beliefs and propositions.
An example of one of my aesthetic reasons. I like irony. The intrinsic ambiguity of human interactions intrigues me. We say or do something, and then later do something else that changes what the first "something" meant or represented. Jesus may have infinite compassion, but he has no sense of humor.
There's a German word of which I am most fond: "vieldeutig." Literally translated: "having many meanings." The English "ambiguity" while signifying "having multiple meanings"
also implies that this multiplicity of meaning stems from a lack of clarity and is a defect. "Vieldeutig" does not imply a similar lack of clarity.
Further, I don't know if laughter without ambiguity or "Vieldeutigkeit" [the noun form of the adjective] is even possible. If laughter at something is forbidden, rational inquiry "uninformed" by revelation is also forbidden. That leads to my issue of values.
I like thinking and looking at things from a variety of angles. It's fun. It's not only thinking that is at stake, but an appreciation of the nuances of (un)certainty goes along with a fondness for Vieldeutigkeit. I don't find belief to be a binary opposition: belief or disbelief. Nor do I think that my subjective feelings of certainty necessarily correlate with the truth of an idea or of a proposition. Belief and certainty are a continuum. The certainty that Christians typically crave strikes me as too akin to an addiction for my taste. This craving is simply too compulsive, neurotic and predictable for my taste.
Appeals to the comforts afforded by faith and to the necessity of a congenial weltanschauung carry no weight with me. It's the timidity and the wuss-factor that turn me against religion. What about courage? What about a modicum of manliness in going out into the world? A good number of arguments "for" Christianity amount to little more than appeals to conformity and laziness.
If Christian apologists are to be taken at face value, they teach that the hardest and most difficult task imaginable is thinking for oneself. In fact, Christians teach by doctrine and example by that thinking for oneself is so incredibly difficult that it is impossible, but if possible, then it is of such little worth as to be safely forgotten, and if not forgotten then deserving of a dreadful and eternal punishment. But--if thinking for oneself is not as impossible or dangerous as they teach?
Christians present the quest for truth as something best finished ASAP. Once the quest is accomplished, one returns home with no further need of arduous labors and difficult journeys. But what if questing itself is found to be an enjoyable and even pleasurable activity? A famous passage from Lessing captures this ethos perfectly:
If God held all truth in his right hand, and in his left the persistent striving for the truth, and while warning me against eternal error, should say:
Choose! I should humbly bow before his left hand, and say, "Father, give thy gift; the pure truth is for thee alone." [Lessing, Werke, Vol. 2, p. 53.]
What would it take for me to believe in God? I would have to be somebody else. Perhaps post-traumatic stress disorder: I suppose I could in principle be tortured & broken in spirit into being a Christian, but I don't think that is what most Evangelicals mean when they ask what it would take.