Assume that all theists take for granted a priori that God exists, and that they have no individual experiences which might indicate that this might be the truth. To whit: they have no gnosis, no "proof" or "evidence" even in their own minds that what they say and do is true. Conversely, all atheists ("without religion"; with this term in this argument, I will consider primarily the so-called "neutrals" of soft agnosticism) do not take a priori that God exists; or, in other words, it must be proven to them a posteriori that God exists.
In such a worldview, it would be impossible for people to "convert" to theism.
Atheistic investigators such as Lewis Carroll come to mind as people who take a priori that God does not exist (null hypothesis), and through experimentation, argumentation, and general experience with Christianity was impressed by its ideals, accounts, testimonies, and so on and so forth. Similarly the story is recounted over and over of a person who lives a "life of sin" and bottoms out, so to speak, in a sleazy motel room. Upon opening the dresser drawer they find a (Gideon) Bible, read it, and therefore "find Jesus." These people had not taken a priori that God existed; rather, curiously they open up a Bible or investigate Christianity, find something they like and that reverberates with them, and cling to it with their new-found gnosis.
I cannot honestly deny that such events happen, that people unlikely to pursue Christianity have some life-changing experience or interaction with God (on whatever level, we cannot decide), and therefore change religions. It happens with virtually every religion, and even happens when converting from one religion to another. The fact of the matter is that there is some personal experience (gnosis) which proves to the individual that God exists, and that that proof cannot be easily transfered or replicated in any other human being.
As far as I can tell, and from all the Christians with which I have spoken, all have had some kind of personal experience with finding God, whether it be in the pews at church, whether it be whilst reading the Bible, or whether it be on their knees praying to God. None of these people would continue to follow their religion were they not motivated by their own personal experiences; countless stories of teenagers who do not feel this same experience and subsequently leave their respective churches proves to me, at the very least, that a priori reasoning is not the reason most Christians stay in their churches. The experiential evidence they have may very well be in assuming some proposition, but that very assumption and the consequences thereof form a basis for a self-sustaining belief system.
The philosopher and psychologist William James once stated that we believe new things we come across simply because they mesh with what we have known before. If there is something so radically different that is being introduced into our minds, we are very likely to reject it simply because it does not mesh with what we already know. Sadly, many evolutionary biologists have used the results of evolutionary science to sneer at Christians for their "fairy-tales," when the stories themselves were not mean to be taken literally in the first place. Despite this, I can still respect some atheist's concessions to the general public; even Dawkins said it would be all right to call "God" the laws of science themselves.
Now, both theists and atheists (a priori positions) put their hypotheses through the experiential process (a posteriori reasoning); some atheists become theists, some theists become atheists, and some atheists and theists stay the same way. However, we should expect some uniformity in human experience, no? It's very apparent that there might be a sampling bias, but I cannot honestly say which side has the sampling bias.