I think for me it was back when I was a teenager. I was raised in a christian family and had very strong belief in it as truth.
I was also very inquisitive however.
I'd come up with tricky questions, that I was genuinely interested in answers for, not just stirring trouble, but when they were too difficult to answer for my parents or sunday school teachers the answer would often come back, "There are some things only God knows. We aren't supposed to know all the answers until we get to heaven..." ie. just have faith.
This answer was the first stage of a dissatisfaction for me. It didn't feel right.
I felt that if God knew the answers, and told them to us with clear enough explanations, that we'd be able to understand it, at least eventually. That the human mind can comprehend anything if given explanation.
Being young I didn't change my life at that point, but the feeling would eventually form the basis of a thought process that lead to my escape from christianity. Blind faith wasn't enough anymore.
