Why be good?

[Thumpalumpacus]

But why? If God does not exist, why be good?

First and foremost, if you have to ask "why", can you even consider yourself a good person?

Think before you make stupid posts.

His religion teaches that he, and all other humans, are "fallen", and unworthy. This is just one more example of the self-loathing inculcated by Christianity.

Thinking is part of being human, it is what separates us from every other living thing on this planet.  If you cannot properly think before stating something, then you must be a theist and/or a republican.  

Forgive me while I point and laugh at these simplistic overgeneralizations. I wonder if you even realize that you've said here that theists and Republicans aren't humans ... or if you understand the significance of dehumanizing your opponent, as has happened through history.

Your shrill lack of nuance undermines what cogent points you might have.

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What I am asking is: what is the BASIS for objective moral behavior?

Morality is subjective, not objective. Your question is loaded, but I'll try to answer it anyway: there is no basis for objective moral behavior.