The scenario of Christian to atheist to Christian again happens but it is extremely rare, even though christians love to tell the story of those that did this. The usual scenario is someone was religious…usually very religious and active in their faith. Then for whatever reason some doubts creep in. Often they go through a long period of trying to resolve their doubts and sometimes do. They never left, they just had questions.
More often, those little doubts grow into bigger doubts. Research and investigation to resolve the doubts fail and at some point, they realize they are atheist. I know many ex religious. It was a process for all of them. No one wakes up one day just not believing in God. For some it takes a year, for some it takes many years. Often, when someone finally declares their atheism, their friends and family have noticed the trailing off of involvement in church and think they were never True Christians to begin with. Yes, they were…it’s just not a quick easy decision.
Most atheists I know…and I know a lot…are more versed in the Bible and their religions than many others in the pews. During their loss of faith, they study it all. By the time they leave, they usually know why they left…the biggest being they just no longer believe it.
The upcoming generations are the ones that will most likely never been raised in a faith as it’s their parents that have already lost it. Some of these will discover religion on their own. Many will never buy into the Christian concepts of faith and dogmas. We don’t know yet how many who’ve never been exposed to faith will later find it but it’s a much longer road than those who were exposed from infancy losing theirs. Someone raised in the secular world of science and skepticism have an extremely hard time with the concept of a supernatural layer on to of the real world we can see and touch…especially with no actual evidence other than other people’s experiences. If they have a supernatural experience, then they have that as evidence. Most never will.
Those within a religion often can not understand why someone else doesn’t “see” their Christian faith like you see it. All religions have their hard parts to explain much less believe in. The less exposure to Christianity, the less sense it makes to outsiders…just as Hinduism makes so little sense to Christians having never been exposed to more than a little bit of it.
As a Jew who lost her religion long ago and tried desperately to regain my faith, I went through a couple of years trying to understand and believe in Jesus. In the end, I think the biggest reason I failed to believe Jesus was God is my Jewish upbringing. Being told over and over that God can not be a man nor can man be a God, it was just too high a hurdle to jump. To already believe in Jesus, the jump from a Protestant faith to Catholicism is much smaller, much easier.
For all the reasons an atheist becomes one, it’s hard to backtrack and believe it again. That’s why atheism to belief is much more rare than belief to atheism. I’m not denying it happens…it’s just rare.