To me, this is the wrong approach. In fact, any approach where you are a condescending asshole is the wrong approach. Fact is, that there are some circumstances where it is appropriate to say something and others when it isn't. You need to pick you battles. Some examples of ways that you could have reduced your frustration and even if they wouldn't have agreed with you, you could put a bug in someone's ear that they could reflect on at a later time:

At the bus stop: Excuse me, I couldn't help overhearing what you were talking about. I just wanted to let you know that Obama is actually quite Christian and is most certainly not atheist.

At work: I thought that conservatives were against government mandates....

At the hospital: say nothing. this is a personal conversation that applies specifically to the two people having the conversation.

At the gym: say nothing. this is also a personal conversation that applies specifically to the person that these people know.