Used to be a Christian?

Yesterday my coworker got a phone call from a donor. I work at a Christian non-profit mission organization. The donor took a while explaining why she was giving to one of our missionaries. She said there were so many people who grew up in Sunday school, who in college decided to leave the faith. She was giving to a missionary to college students.

My question for anyone in the world who will leave a comment-answer is this:

If you ever used to go to church or believe in God or Jesus, and you don't now - why? What were your reasons for quitting your faith?

I'm asking as a non-judgmental, curious Christian. Yes, we exist.