I wouldn't say atheists are retarded.
I wouldn't say Christians are retarded.
I wouldn't say agnostics are retarded.
Nor any other religion. In my opinion, people can believe whatever they want. However, if you're an atheist trying to tell people of other religions that it's all bullshit or a Christian screaming at people how they're going to hell unless they repent and turn to the Lord for help, THEN I think you're retarded.
Everyone should stick to what THEY believe in and stop bashing other people's beliefs, or lack of beliefs. And no, atheism isn't a religion. So stop being idiots.