Dr. Ruth Tucker, an evangelical and a former professor at Calvin Theological Seminary, wrote a book entitled: Walking Away from Faith: Unraveling the Mystery of Belief and Unbelief. I have the book and have read it with great profit.
In a 2001 lecture to the Freethought Association of West Michigan, she gave 5 myths about those who walk away from their faith.
1) They are angry and rebellious.
Tucker says that her studies do not show this to be the case. Rather than anger, deconverts are more likely to experience sorrow and pain when they initially depart from the faith.
2) They can be argued back into faith.
She says this is very rare because the person leaving the faith "has carefully and painstakingly dissected the reasons behind this major worldview change." Committed Christians do not leave their faith without a lot of thought and soul-searching. Its not something that is done quickly or flippantly.
3)Doubters can find help at Christian colleges and seminaries.
She didn't find this to be true.
4)They abandon their faith so that they can go out and sin freely.
She found no evidence for this contention. As a matter of fact, she points out that often those who do not have faith appear to be more moral than those who do have faith.
5) They were never sincere Christians to begin with.
She rejects this because in her research, she came "across example after example of the most earnest and devout of evangelical, fundamentalist believers who became non-theists."
Evangelicals have a really hard time explaining why one would depart the faith. Most that I have encountered since my apostasy seem to opt for either reason #4 or #5. They say either I wasn't "really saved" to begin with or that a desire to commit certain sins drove me away from a belief in God. They just don't seem to be able to admit that someone who has studied the Bible carefully, and understood its teachings as they do, could possibly leave the faith for intellectual reasons.
Tucker went on to list some of the real reasons people give for leaving their faith.
1) The study of science & philosophy
2) The sense of absence of any caring God
3) The critical examination of the scriptures.
4) Disappointment in God
5) The hypocrisy of Christians
6) The perception of a dogmatic anti-feminist and anti-homosexual stance of fundamentalist Christianity.
In my case, I would say that it was primarily #3 and to a lesser degree #5 that led to my de-conversion. I would be very interested to hear from other apostates as to why they abandoned their faith.