In 2016 a Pew Research Center survey received the following reasons Christians abandoned their faith:
Learning about evolution when I went away to college.
Religion is the opiate of the people.
Rational thought makes religion go out the window.
Lack of any sort of scientific or specific evidence of a Creator.
I just realized somewhere along the line that I didn’t really believe it.
I’m doing a lot more learning, studying and kind of making decisions myself rather than listening to someone else.