Action! No matter what kind of situation it is, people can talk all day about the good they do, but until I see it, its just that, talk. In my experience, it helps also 2 not just blurt out right away that u r an atheist/agnostic or whatever. I usually just do things that make me feel like a good person. If others see that and think I'm a good person, cool, if not, no worries. I still know. And most of the time, at least with the folks I've met, once they get used 2 thinking of u as a good person, when they find out what u r, they might b surprised but they're usually not 2 freaked out.

I don't give a rat's ass what they think about me

The idea is subjective, so the answer is also.
The best way would be to find common ground the spread the common ground so wide that they have to see their previous concception is necessarily changed, but they could just ignore it and revert.

Like with all things: through your actions. Some are so ingrained in religion that they don't possess enough critical thinking skills to move beyond what they've been programmed to do. Because of this, fear of damnation is what they need to stay in line ...and honest.

The only way for people to accept you as you present yourself is actions. If you are trying to convice people with words you are wasting your breath. Actions show who you really are.