1) A crisis of some kind that cannot be reconciled with one’s faith. It could be the loss of a child to Leukemia, being molested by a priest, or a tornado that ripped through one’s house. It could be bankruptcy, or being sent to prison when innocent. Or it could be the loss of one’s life savings because of a con-artist. It could be noticeable hypocrisy from church members, a church fight, and/or being ostracized by the church rather than cared for in times of need. The church is, after all, the one place where they shoot their own wounded. And when cast aside by the church these believers must deal with any doubts on their own, and at that point they are free to question what they believed without the social pressure to conform. Many different tragic events could cause Christians to question their faith. While most Christians will reaffirm their faith in the midst of a crisis, many others will reject it. Then it will be said of these former believers that they left the fold because of emotional reasons, when the fact is that the crisis merely shocked them into doing what sane reasonable adults should have done all along, questioning what they believe in the first place. Snapping is a good term to describe this process, for in a very short amount of time a person is snapped into seeing these things in a new light.
2) An intense desire to know the truth objectively as possible. This is the intellectually honest approach that few believers ever even consider doing. Having been raised as Christians in a Christian culture they seek as adults to re-evaluate what they were taught to believe because they have subsequently learned that their parents were not always right about everything. This is where they attempt as best as they can to be consistent with the various religions and sects in the world by subjecting them all to the the same level of skepticism. This represents my Outsider Test for Faith that I’ve been defending here at DC and in my books. At this point they will engage in a real quest for knowledge by reading both sides of the debate, which can and does lead to skepticism about all religious faiths. I have a challenge for these kinds of believers called the Debunking Christianity Challenge. Take me up on this. I dare you. It can do you no harm. Quite the contrary, it can only make you better informed.
3) A deeper study of one's inherited religious faith. Knowledge is the antidote to the delusion of faith. With childhood indoctrination having such a powerful grip on believers, as I wrote about here, containing so many powerful ideas to keep them in the faith for as long as they live, it sometimes takes a massive amount of information to overcome it. There are people driven to know what they believe and why they believe it. They did not expect to find there was no basis to believe, they just wanted to know about their inherited faith. They are thinkers. They want to know. They have an intense desire, more than most, to become informed. They are not afraid of coming to their own conclusions despite what others whom they know and respect believe. These thinkers are so passionate they enter college and get advanced degrees in theology, Biblical studies, philosophy, or science. While most believers are satisfied with the first glib answer they come across, or they simply punt to faith that God knows the answer, these thinkers are not so easily satisfied. For Hector Avalos, Robert Price, Bart Ehrman, William Dever, and many others their quest for more knowledge ended their faith, as it did for me. In a similar manner a deeper study of one or more intractable issues that cries out for a solution can and has led believers away from their faith. For the late Ken Pulliam it was the atonement that led him away, even though all he wanted to do was to solve this problem. He could not make sense of how the death of Jesus on the cross could possibly save people from their sins. For exapologist it was a deeper study of the promise of the return of Jesus, which he could not make sense of except that it was a failed one.
4) Entering certain occupations leads Christians away from their faith like the ones listed here.
5) World travel, not as a missionary necessarily, but just world travel, lots of it. Then it will slowly dawn on the Christian that there are happily fulfilled people around the globe who are both morally good and rational even though they were raised with different religious beliefs. This can be a shocking experience. It will teach Christian believers how parochial and sequestered they are from the real world, one that gets along just fine without any direct Christian influence or teaching. Reading world literature can also have the same effect as traveling the world, for it's exposing oneself to the people of the world at large, their morals, politics, religious beliefs and social customs.