Many non-believers used to be believers. It's not a matter of free will, which I don't think we really have, but exactly like the Santa scenario, where if you believed as a child, could you ever go back to belief in Santa? The answers would be no and it is likewise for magical Who's, you simply understand enough about reality or begin to see reality better where beginning or going back to belief is impossible. When it comes to believers, there is an irrationality that creates a bit of a willful ignorance to better reasoning in order to protect the belief for emotional reasons. Once a believer intellectually wants to make sense of their belief and religion of choice, that is usually when the breadcrumbs back to reality appear and they simply follow them right out of their faith. Another breadcrumb is the fact that theists often struggle with faith that comes and goes like the tide. If there was better support, there would certainly be less of this happening.