Daniel Dennett recently finished his study on clergymen who have become non-believers while still being the pastor of a church. I read the paper yesterday and it was quite eye-opening to read about their mindset and how they justify their continuation of pastoral duties while not believing the words they preach. I don't want to spoil it for anyone who might want to read it (which I recommend you do) so I won't talk about the details of the paper. The de-conversion process for all of the 5 clergymen interviewed was a gradual process that traced itself back to when they started to learn more about their faith, and for most of them started when they went to seminary. I suspect it is the same for most people who leave their faiths. Not many people wake up one day and say to themselves, "Ok, I don't believe this anymore".
The Study is available on the Washington post's website HERE