Can't remember one single lightbulb moment. My father had a catholic upbringing, my mother agnostic and I went to a 'high church' Church of England school, so God figured a lot. I remember believing then (say, under 14 yrs). It was more an accumulation of thoughts over the years, the increasing evidence (as it seemed to me) that the universe, and we, had come about without a 'prime mover'.
The tipping point was reading Genome, by Matt Ridley. I am not a scientist, so learning about the complexity of our DNA made it impossible for me to believe that we had been designed. Have also read Dawkins' God Delusion, although I dislike his need to tear down faith quite so aggressively. Unlike him, I think humans are essentially tribal, and if we weren't killing each other over religion, we would find something else to do it over.
I also have a couple of 'actively' Christian friends. A huge part of me envies the comforts of their faith. I also love Christian music and find churches peaceful and good to sit in and think. But really really wanting something to be true, doesn't make it true in my head, so I just can't get there (to belief).
I also don't think that religion has the monopoly on morality, far from it.