Is it wrong to tell other people that their beliefs are wrong, that there is probably no God, and that they are wasting their only lives on a false belief? Some people would say "yes". In fact, most people would say "yes", including some atheists.
Let us consider some of the arguments against preaching atheism: (1) atheism is just your opinion, religious people have their own opinions, you do not really know who is right; (2) people should keep their beliefs (or lack of beliefs) to themselves, it is not right to push your beliefs on others; (3) religious people are happy, it is not right to take away from their happiness and make their life confusing.
The first objection is one that I have to deal with on a day to day basis. Every day, or almost every day, I have to hear people remind me, "well it is just your opinion", not just on the whole religious issue but on every other issue. I hate it. I hate it because it is an anti-intellectual mentality. Everyone has opinions. So what? It does not mean all opinions are equal. If you have some pain in your stomach and you say "perhaps I should go to the doctor", and I say, "you have a tree growing inside your stomach from all the seeds you ate" then it is clear you do not need to take me seriously. We both have opinions. I had a pretty retarded one but it was an opinion nonetheless. (I blame this anti-intellectual attitude on college. In college you often hear professors tell you how you need to respect other's opinions. But that is another issue that is taking me off topic). Opinions need to be challenged and argued for, the best one should win out. So this objection to preaching atheism is a bad objection, not to mention self-defeating, if people have opinions and they should not be taken seriously, then what you just said is an opinion, but somehow I have to take it seriously? It just makes no sense. This argument is not going to stop me from preaching disbelief and heresy to other people.
The second objection I disagree with. Who said that people should not push their beliefs. (College again?) I hear this all the time, on TV, on YouTube, on online discussions, we should not push our beliefs. But why not? I never hear people say a reason for not doing so. I agree that it is hypocritical for atheists to tell theists "keep your religion to yourself" but at the same time post bus posters in promotion to atheism. But I do not think that religious people should keep their beliefs to themselves. Let us say that you believe in Jesus. And you think Jesus is the greatest person to exist. And you think that Jesus is the truth and the light. And that Jesus will make your life and the world a better place. If you believe in all of that, honestly believe in that, then what kind of a person are you if you keep that to yourself? I would consider you a rather terrible person. You have this great gift that you can share with the world and you keep it to yourself? If you really think that Jesus is great then tell other people about it. Of course, be nice about it, but preach it. There is a difference between preaching your beliefs and enforcing them. I agree that people should not enforce their beliefs, after all that would violate the separation of church and state. But preaching is fine, I encourage it.
The logical conclusion from the third objection is that if drug users are happier using drugs then we should not try to help them and just let them continue taking drugs because otherwise we would be interfering with their lives. Of course, no one agrees with this, at least I hope so. We got no problem with telling drug users that they are destroying their lives, they can live more meaningful lives off the drugs. Religion is not that different. Religious people are destroying their minds. They destroy their individuality by being slaves to an authoritarian God. What kind of people are we if we let them live a delusion and self-destruction? I am not saying that atheists should deprive religious people of their right to be religious. I am just suggesting that atheists should reach out in some way and let the message of a secular life be known. So that maybe some people would listen and change themselves (and the world consequently) for the better.
I once, like a year ago, was searching craigslist under personals. I searched under the title "frum". There were quite a lot of frum gay Jews there looking for gay sex. I responded to one of the ads (if you are wondering, no it did not get anywhere). I send an e-mail saying that he does not have to live a depressing life as frum and gay, I said that the arguments against religion are too strong, if he really studies it honestly I am sure that he would leave religion, and thereby attain a happier life for himself. He got very angry at me in his response by e-mail. Did not speak with me again. But this story does not bother me. Because sometimes when you try to do good to others they will be angry at you for doing this. Their anger and hatred to what I do will not stop me from doing it. Because, in the case of frum and gay, I think that if they really do consider the secular lifestyle then they would have much better life. Lives that they cannot have under Judaism. Lives that they are deprived off and need to live in depression denying such a life. Their resentment to me is no argument against preaching atheism.