I was raised as a Protestant Christian. My family is filled with Biblical literalists, and I was raised as such.
As I grew up, I saw a mountain of scientific evidence, observable facts, showing that the Biblical account of history could not possibly have happened without God going through a great deal of trouble to cover it all up. It just seems dishonest for God to create the universe so that it looks, for instance, to be a certain age by all manner of evidence, and then tell us that it's not. I also saw various Christians behaving in a very un-Christ-like manner, using biblical passages and a literal interpretation to justify all manner of horrible crimes. I saw that the hyporitical religious leaders of Jesus' day are still present today.
I realized that I couldn't take the bible literally. I still believe in God, but I believe Him to be the loving, merciful God described by Jesus. I believe the atrocities of the Bible were attributed to God by men who needed a justification for their evil actions, and knew that no one would speak out against God's Will, whether it really was His Will or not. Just like Islamic terrorists, or Christians who murder abortionists.
So science and atrocities in the name of religion are what swayed me from my literalist upbringing. The root of my beliefs now is a faith in a loving, forgiving God. It's not rational - I have no real evidence for His existance, and I believe the Bible to be just an old book with some really good lessons mixed in with the rest. I choose to believe in Him only becuase of personal experiences, and the fact that the
good lessons of the Bible are undeniably real and good, even if the events never happened.