Most Helpful Guy
Unfortunately a trend among many atheists these days. They lack respect towards people who don't follow their belief (or rather non-belief).
It is more of a problem of his character though which you should put into question when he behaves like this to his partner.
Most Helpful Girl
Don't try to be more harmonious, because it sounds like you'd have to pander to him rather than simply be mature. I guarantee that if he did follow a religion, he'd still act the way he does now, just with a different excuse. It doesn't sound like he's willing to accept or respect your religious beliefs, which means he is the cause of the problem here, not you.
Honestly, I'd suggest breaking up. This might not the biggest deal right now, but I'm willing to bet that later on it's going to become a major problem. It even sounds like he's beginning to lose respect for you. I'm positive you can find a guy who's at least willing to respect your beliefs, if he doesn't share them.