I think it's safe to say ™ and/or a huge majority of people who identify themselves as Christians are merely people who adopted the culture they were brought in and that culture included religion. That said that big group of people do not strictly follow the bible for instance in a big city such as new york I believe it would be near impossible to find one practicing Christian who doesn't cut his hair, doesn't wear two different cloth fibers, and all the other doctrine that most Christians say they abide by so it's only the extremely fundamentalists that are true Christians if you think about it that way. So why isn't ever Christian that faithful? Because deep down all of those people have doubt and/or a sliver of acceptance about the reality of the world we live in. You simply can't live a True Christian lifestyle and not back track on a lot Rights, Liberties, luxuries, and freedoms that this world holds. Christianity is just one religion among hundreds and even in Europe they have increasingly have non-religious people become the majority when they were the country who brought the Christian religion to America in the First place and Created the Church of England to allow divorces. So to end, with the passage of time even a religion you may hold dear whether it's Christianity, Buddhism, Judaism, or Islam the world will change, grow, and become more educated to that absurdity of it all.

I think it's more accurate to say non-theism predated religion. Atheism is just a rejection of the belief of all religions. Non-theist eventually explained things with deities, gods, whatever supernatural beings and created cults/religions and some people disagreed with their ideas and became atheists to that particular religion.