Politics and religion are often subjects that are dicey to discuss, at least in America. People tend to have strong opinions on these topics, and arguments can get heated. That's why it's considered a breach of etiquette to post polarizing political or religious statements, like "The President is a moron!"
When you post something like this, two things will likely happen. First, you'll annoy many of your friends. Certainly the ones who disagree with you, but also people who might agree with you, but not the way you're putting the message out there. Second, you'll ignite a firestorm of nasty comments, as those who you've ticked off begin furiously posting angry rebuttals. Soon they'll turn to bashing each other, too. Is this really what you want?
If you're passionate about politics or religion and want to discuss certain topics, that's fine. As long as you post thoughtful, polite, well-reasoned comments and respond respectfully to those who disagree, even if they post a harsh reply. But think about it for a minute. Have you ever seen a thoughtful, respectful political or religious discussion on Facebook? Exactly.