* 14.89% were dissatisfied with some of the answers given to them by priests or other religious people.

* 14.89% saw that the religious doctrine wasn’t compatible with science and reality.

* 12.76% came to the realisation that religious dogma was “internally incoherent” and illogical.

* 10.63% read the Bible and found that for the Holy Book of Christianity, it didn’t reflect modern day Christianity at all.

* 8.51% found the corruption and scandals following the church as a persuasive argument against following their doctrine.

* 8.51% gave up on prayer and religion when they realised that nobody was going to listen, let alone answer to them.

* 8.5% thought the similarities of Christianity with so many other religions (and yet so different at the same time) the primary factor of their deconversion.

Source

http://kieranbennett.com/index.php/2008/05/12/what-works-in-deconverting-christians/