In What Other Civilized Nation Would This

[Hatshepsut]

Marco Rubio's Presidential Campaign: Five Facts About His Faith [in Huff Post]

I'm a bit leery of the Huffington Post as a source. They're loose with their facts. For instance, the cited article suggests incorrectly that Mormons are enjoined from working as bartenders when they are only told not to drink alcohol. Lots of bartenders don't drink; lots of Mormons own restaurants and bars without damage to their church standing. Nor is it too easy to determine what Rubio thinks of atheists: Could he be pandering to his right-wing flank by rushing to affirm he is faithful? I'm not aware he has said much about atheists himself, even the Huff Post acknowledges the stuff on his position comes from bloggers, not from the horse's mouth.

If a guy in my country said atheists are less good citizens I could sue him for defamation...

There may be good reason such grounds for a defamation lawsuit wouldn't fly in the USA. A statement that "atheism makes X a less desirable citizen" is too vague. You can indeed sue for defamation of character in America, but you must point out a specific lie or misrepresentation and show that actual economic damage resulted from it. A hiring blacklist, for instance, might be a target for a defamation suit.