My father is not an atheist. He's been a Christian longer than I've been alive. While his attitudes and behaviors have certainly moved away from what most Christians would recognize over the years, he still considers himself a Christian. I suspect this is largely due to a combination of the manner in which he was raised, his distaste for getting bogged down in deep questions, and the psychological and social benefits he accrues from Christianity.
When I was a child, one of my father's expressions I always liked was his description of church as "a waste of a perfectly good Sunday morning." He never seemed to enjoy going to church, and I think he went primarily because my mother wanted to. He may have shared her belief that church was good for me in some manner, but she seemed far more convinced of this than he ever did.
My father's idea of a perfect Sunday seemed to involve getting up early, doing whatever work around the house needed to be done, playing a competitive game of tennis with a friend, and then watching a football game or two. This is what he would do when given the opportunity because my mother was sick, out of town, or there was some other compelling reason to skip church.
At the point during my late teens when my mother finally gave up on forcing me to attend church with them, I can remember my father asking, "If he doesn't have to go, why do I?" It was not long after that - maybe 4 years - that they both stopped going regularly. They would go for holidays, but that was about it. A couple years later, and that would stop too. Neither of them seems to miss it one bit.
My father's ideal Sunday morning looks a bit different now than it did all those years ago. With retirement, weekends do not hold the same significance they once did. There's less to do around the house than there used to be. Fishing often calls, and there are always friends with free time who are interested in doing something. He's still very active, but he gets to spread it out more now since he has more time. And my father is completely free from church now. No more wasted Sunday mornings.