Unless it's brought up by the other party, I just don't offer an opinion or stance.

If the other party brings it up, but in an accusatory, close-minded, or even disgusted tone then I also don't engage or offer an opinion or stance. My parents and my roommate at school are the type who hear you don't believe in a higher power and automatically write you off as one who has no moral compass, no guidance, and they see you as the Bad Guy. They're incapable of having a mature conversation about opposing views, so I don't waste my time trying to explain my beliefs, and I never ever bring my personal beliefs up voluntarily.

It's difficult. I want to be angry and argue and debate. But you have to remind yourself that if nothing they say in favor of Christianity is likely the change your mind, then nothing opposing it is likely to change theirs. It's so easy to be defensive and we should stick up for ourselves and our beliefs - but you have to choose whether causing a riff between you and your loved ones is worth it.