But there is a fundamental problem with this question.
Before I get to the problem, I wish to make it clear that what the religious mean by belief in this context is nothing other than faith - which is to believe in a given statement even if there is nothing in reality that supports the truth of that statement. In other words, faith is belief in a proposition not backed by evidence, and when the religious talk about belief they often mean faith.
That should point to the problem here. Intellectual atheists, or more accurately, skeptics demand evidence for claims you make about reality. If you make a claim, a skeptic would ask you to present evidence for this claim. A skeptic would verify the authenticity of the evidence, and also verify whether the evidence does indeed support your claim instead of doing nothing, or worse, refuting it.
That's where the problem lies. To ask a skeptic what it would take for him or her to become a believer, would be to ask him to put aside his skepticism and make an exception for whatever it is that the religious is calling him to believe in. But the dilemma is, it takes evidence for the skeptic to accept the claim you make - which in this instance would be to believe in god, or more specifically to believe that god is real. The claim you make here is that your version of god is real, that this god exists in reality.
(For simplicity, I will not touch on "belief in a religion", but this can just as easily be extrapolated for that as well as beliefs in a myriad of things usually demanded of skeptics.)
A skeptic would naturally require evidence supporting your claim as he would any other claim. Assume a hypothetical scenario where you as a believer are able to provide evidence supporting your claim. Contrary to what you think, this would not make the skeptic a believer.
Instead, it would demonstrate that faith itself is not necessary.
If you are able to present evidence to support your claim that god is real, you yourself need not be a believer - because evidence speaks for itself. The skeptic will continue to remain a skeptic, but he will simply accept your claim that is now backed with evidence. You on the other hand, will no longer need to be believer, and by seeking evidence you have entered the road to skepticism.