It's a touchy subject. The frank position of Christianity is that such people are in denial of ultimate Reality (God). Some atheists return the complement to Christianity in mirror image, though positing a reality smaller than God.
Christianity replies once more by pointing to a moral problem. Who to obey and how? Yet God answers that question TOO well... a moral code we observe that our best efforts fall woefully short of, coupled with a hair trigger of blame and the displeasure of an Infinite being. So, many shy away. I did. Until I saw salvation enter the picture. One can't truly be forgiven until one pleads guilty.
Begs so many questions. If one applied the same standard of proof to the existence of God one uses for other purposes - UFOs, fabulous offers through the internet, etc., one would surely conclude there is no evidence for a deity. One might be on stronger ground is arguing there must be an 'uncaused cause', an origin; but it's implausible that the mere existence of a deity as origin can be used to distinguish between the validity of all the various systems of religion out there. As for obedience; even if a supreme being exists, one asks why the being would want a particular pattern of behavior from us, or why we should comply? As for the idea of a moral code which we are doomed to fail to satisfy, and therefore we need to throw ourselves on the mercy of the court, what a truly twisted way to look at the universe! Wouldn't a benign deity set realistic expectations for his underlings, as any good human manager would?
All of it is irrationality heaped upon irrationality. It's the kind of thing a small tribe adrift in a hostile and inexplicable world would dream up as a cosmology; come to think of it, it's the kind of cosmology such a tribe did dream up.