According to A.A. Milne
(he wrote Winnie the Pooh
The Old Testament is responsible for more atheism, agnosticism, disbelief, call it what you will, than any book ever written. It has emptied more churches than all the counterattractions of cinema, motor bicycle and golf course.
The Old Testament. It's where we learn that you must stone to death a man wearing clothes of two linens, where we receive all the instructions for buying and selling slaves... and where we are subjected to all those boring family trees!
And when your average Joe off the street picks up a Bible, where's he gonna start? The beginning! Sure, it starts out easy with the familiar stuff: Garden of Eden, Tower of Babel, Moses, et al... but once you get out of Exodus you start dealing with some dry and outdated stuff. An evangelist once told me I should read the Gospel first. Shouldn't The Good Book that controls the lives of millions of believers all over the world be inspirational cover-to-cover?
A Christian friend of mine defended the Old Testament by pointing out that God imposed several rules for health reasons. The no clothes with two linens rule, for example, was based on diplomacy with the nations surrounding Israel. There were several rules that made it unclean to touch lepers and other forms of nastiness. I found it odd that the same God Almighty that dropped food from the sky couldn't include a couple of vaccinations for his chosen people in the scope of his interest, in lieu of having them put to death anyone who unknowingly subjects himself to disease.
I may not speak for all the atheist everythingians, but I claim the Old Testament as the greatest insult to reason (q.v. MoJoe's node) and therefore the single greatest cause of atheism.
Disclaimer: This node seems to me to be a fairly casual forum for opinions. I am not trying to categorize all atheists and/or flame Christianity; I'm just contributing my $.02 as far as, well, what I think the single greatest cause of atheism is.