I am an atheist. I'm proud of this fact, and also pride myself on generally being a fairly rational human being. While I'm fairly set in my religious beliefs, I am open-minded in that the sudden appearance next to me of some divine being (whether a Judeo-Christian God, an invisible pink unicorn, or whatnot) would probably cause me to believe in them, if it wasn't just some hallucination or otherwise a deception. This is an important point. While many theists are sensible folk, there's some who would not be willing to revise their beliefs in the face of any evidence whatsoever, no matter how much it proved them wrong. Such people are really not worth dealing with.
Also unlike your average person, I tend to be involved in a fair amount of philosophical discussions, and a significant fraction of these involve the nature of reality, ourselves, and any possible divine entities. As anyone who's ever had such a discussion can attest, very few people ever change their mind as a result of one, but nearly everyone seems to evangelize on behalf of their belief system. Whether this is because we feel everyone else would benefit from thinking like us or because we crave the justification of convincing others we are right is hard to say.
I feel I can safely say that convincing another individual that god doesn't exist is next to impossible. You can set them on the right track for coming to this conclusion on their own, but it's a journey they have to make themselves. Furthermore, attempting to convince a believer that they are wrong is futile. Even intimating that they are wrong trips an automatic psychological defense mechanism, and the other person will become increasingly hostile they more you assault them, even if they're normally a calm, mild-mannered wimp. Therefore, I've adopted a very different strategy in spreading the thought (words seem to be much easier to disseminate than thoughts, probably because most people do much more speaking than thinking). In such discussions, I generally wind up taking the role of what I refer to as Heaven's Advocate (by a devilish little analogy). My point is to prove that rational, scientific thought is not incompatible with a belief in some form of god or with the bulk or traditional religious morals. (Note that I'm not saying that rational thought leads to belief in gods, only that it doesn't necessarily preclude them, as long as they retreat somewhat from the realm of direct involvement in the day-to-day affairs of our world. Your average Christian reacts fairly positively to such a strategy. (They constitute the vast bulk of the theists I encounter, but the same tactics apply to any monotheist; polytheists are a different lot entirely, and tend to be far less fundamentalist and obnoxious.) The fact that you are defending them against the hardliners of your own cause immediately generates sympathy. More importantly, you show to them they don't have to give up their beliefs in order to accept rationalism. Once they actively engage on the path to rational thought, they often end up in the state of continuing to believe in a god, but a rather distant one that doesn't care what religion you are, only about how you behave in life, and that created a mechanistic universe and then pretty much left it to its own devices while observing from afar. That's really quite decent. An occasional person will go all the way and convert to atheism, but doing so requires letting go of many mental security blankets, and most people aren't willing to do that. Still, if you can convince just one person to start thinking, then it was worth it.
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