This post is going to be quite a diversion from my normal style, discussion, and focus. I'm going to offer tips for atheists here that are merely my own opinion. Some atheists will disagree with many of my claims, but this is just what I think based on experience, hard thinking, and reading.
This post, in no way whatsoever, is meant to be elitist or condescending to theists. Not all theists are bad debaters who are afraid of discussion or people who resort to personal attacks.
How much is too much?
At some point, discussions will get nowhere. Some theists will propose plausible arguments and a substantive discussion will be had...but if the theist is not willing to consider arguments and automatically dismiss everything I have to say, the discussion is pointless. To have an open mind, you need to consider, acknowledge, and rebut points made by your "opponent" instead of not considering the arguments, bypassing them, and bringing up new claims.
Should atheists be public at all?
Yes. Atheists really should be public. The more public atheists are, the more people will understand our views, erase their biases, and think about their beliefs.
In some cases, unfortunately, atheists need to stay silent to protect their jobs, housing situation, placate to parents who would be hostile, etc. It's too bad that this is the case. Discrimination and intimidation from religion is tremendous. Simply making a case against religion is grounds for a mass of people to dislike you, cease to be friends with you, etc. I've received a tremendous amount of hate mail, vitriol, and hate because of my very public stance. There's no doubt about it...
If atheists are going to remain silent, the discrimination, stigmatization, and misconceptions will continue. Stand strong and don't give in to intimidation. Speak for those who can't speak.
I know various atheists who can't speak out because they fear (with good reason) that they may lose their jobs, the care of their parents, or the respect of family members. I hope I can speak for them! Please so the same if you can...
Should we just leave "let people alone?"
I suppose prudence can be a good thing in many cases. Totally "letting people alone," though, would equate being silent, which is not good for reasons mentioned above. I don't recommend, even for a moment, to challenge every religious person you meet or know. We should not, though, fail to challenge religion or have discussion when other people are conversing about religion and you feel that your input is warranted...especially if they say things about atheists that are nasty or claim superiority...more on this below.
Should we challenge religion if it's mentioned?
If someone is having a casual discussion about religion and he/she is making non-absurd claims, perhaps you should leave the discussion alone.
Example:
"I went to church yesterday"
"I gave up meat for lent"
I suppose you can inspire conversation -- if you feel -- and say something like, "I don't understand this belief, why do you do these things?" or "Can you tell me more about that?" that is not antagonistic.
If, though, someone says something antagonistic or dangerous like:
" All atheists are immoral people"
" I don't believe in medicine because the Bible tells me that I can pray"
then you should say something.
If someone else opens the conversation and asks you questions, by all means, have the conversation.
I wouldn't recommend being antagonistic. These discussions often turn into emotional discussions if people haven't discussed religion with atheists before. You can't do anything about that besides saying, "I'm not trying to attack you, please don't feel that way! I'm just questioning your claims and having a discussion."
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What about the internet?
As far as the internet is concerned, feel free to go all-out with a blog, Facebook, or any open discussion forum. If people don't like what you have to say, they certainly can choose to cease viewing your website/profile, un-friend you, or block your news feed. Discussing religion on Facebook will cause you to lose friends, but you'll probably gain some in the process and start some good debate!
Profile Hunting
What if someone else posts something on his/her wall like "God gives me the strength I need to make it though the day," should you post something?" I may go with the same advice I posted above regarding face-to-face discussion: if the claim isn't antagonistic, leave it alone. If the claim is antagonistic or dangerous, say something.
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Tips
Don't Be Hostile
Again, religious discussion is often an emotional discussion with people who haven't heard atheistic arguments. Sometimes there's no way to avoid perceived hostility when you challenge beliefs.
Suppose a theist says this:
"The story of Abraham and Issac was a great story of tremendous faith"
and you say...
Hostile Response:
Wow! Are you kidding! Faith is stupid and any father who would murder his son is a terrible person, your God is evil and you are immoral for worshiping him!
Non-Hostile Response:
Do you really think that it is a moral action to strap your son to an alter and kill him because you think that God told you to do so? Do you think that a moral god would actually ask someone to kill his son? And what's so good about faith anyway? What good is believing in something that is not supported by evidence and is contrary to evidence?
If you're going to be hostile, you're going to turn people off very easily and not have a substantive discussion. While the atheist will certainly think that Abraham was a terrible person for doing this and the story is nonsense, the atheist should be less hostile and use the second non-hostile approach.
Don't Be a Jerk
It's obvious that in discussions with theists and atheists that both sides think the other side is wrong. You shouldn't outright say "You are wrong," but rather have a substantive discussion and make solid arguments. Life mentioned above, you're going to turn people off if you're hostile.
Don't Think You're Going to de-convert people
Many theists have worshiped God for their entire life and have been indoctrinated. Many haven't really researched their position and will not want to change it. Some will say things like "no matter what you say, I won't change my mind." Offer the arguments and have the discussion. Reinforce, though, that your position is the product of logic, reason, research, discussion, etc and dispel the theistic biases of "You're just acting out," "You just hate God," or "God did something wrong to you and that's why you're against him." Reinforce the importance of research, independent thought, investigation, and making your own choices independent of what others say.