Only one religion that everyone has to obey...
No religion at all so people can do what they feel....
How about just a religion of peace, respect, and human rights?
Why has so many people been killed I wonder, oh, because they had a different religion compared to others. Oh!
Hitler was a christian and his killing of the jews was a direct result of christian against jews that had been building up for centuries. Stalin may have been an atheist but he was also crazy paranoid, killing anyone who might stand against him. He also developed a religion like worship of his personality: http://en.wikipedia.org/wiki/Stalin's_cult_of_personality. This was possible because after hundreds of years of being ruled by tsaars who always claimed to be inspired by god, russians had no idea what good rule looked like. The Chinese had been ruled by emperors who claimed to be the direct descendants of god. If people complained about silly things like starvation they should consider themselves lucky to walk on the same earth as a god. Everything was sacrificed for the emperor, including China's military development. While Japan bought guns from the west and so was able to conquer China and most of asia in WW2. They did so and committed many atrocities against the asians because they believed to have divine rights and to be better people. Mao led the rebels to win back China from the Japanese. The bloody history lead them to believe all religion was evil and had to be crushed, which lead to the oppression of religion and conquering of Tibet. Which was lead by buddhist monks. Time and again atrocities were committed because of beliefs, not common sense. Anyone can believe stupid and harmful things, being an atheist just free's you of the god beliefs.
'Only one religion that everyone has to obey' would not leave you with any freedom of religion (which I am not willing to give up), while 'No religion' (in my interpretation) means that religion has simply never been 'invented'. And seeing, historically, how many atrocities were committed because of religion, I think that would be beneficial. I would be OK with a world where everybody keeps their religious beliefs to themselves and doesn't try to force them on anybody, but as long as religion exists, that is unlikely. Also, most Nazis were religious, and Stalin (like other communist leaders) had a religion-like personality cult around himself.