"I don't believe in God". Sometimes, that statement (or its direct opposite) can get people hot under the collar. Regardless of which "god" the statement is referencing, everyone has some sort of a belief system (a religion) they ascribe to, even if it is to say that god does not exist. In a world where people are killed for the most simplistic of things, it is important to understand how we can be accepting of another's religion, without necessarily accepting their religion as true. You'll need an open mind, patient heart, quiet tongue, and a peaceful spirit.
First, take a breath. You believe what you believe, and you probably think that precious little will change that. Realize that other people feel the same way about what they believe. From that realization, you can see that very little of what you say will convince someone their religion is wrong. Getting angry certainly won't help.
Second, do what you can to find out about their religion. It is an uncommon thing for people to listen instead of talk, but ask genuine questions. Avoid snide remarks or sarcastic replies; ask in all sincerity, seeking knowledge rather than a "victory". Respond to their questions and statements with honest answers and positive replies, indicating that you understand them and do not want to offend them.
Thirdly, once you have discovered what they believe on a certain topic, begin a legitimate conversation. If you are meeting that person for the first time, perhaps religion is not the best subject, but if it comes up, talk about it briefly, then change the subject. If you have come to disagreement with a friend, understand that you are still friends, and can speak to them a bit more candidly. If you see a hole in their belief system, do not exploit it with a snide attitude, but ask them how they can believe what they do despite evidence to the contrary.
In all things, remember to give the respect that you expect. Remember to treat everyone with common human decency. No one is less of a human being because they have a different belief system, but that does not mean you need to tolerate their actions. If you believe in a god and another person prays to that god's enemy, you can still politely either remove yourself from that person's presence (and therefore maintain the social peace) or ask that person to observe their religious rituals later (risking a disagreement, but still maintaining tact).
Education and polite discussion is key. Learn as much or more than you teach, listen more than you talk, and-in keeping with your religious beliefs-pray about it, if you are so inclined. If you believe in a god, perhaps that god will help you in your disagreement with someone else's religion. Do what you can to maintain the peace, but never apologize or back down for what you believe, unless you are proven wrong beyond a shadow of a doubt. Remember: everyone could be equally wrong as much as they could be equally right.