NOTE: THIS IS NOT MEANT TO INSULT ANYBODY OR CALL ANYBODY WRONG. EVERYBODY HAS THE RIGHT TO BELIEVE IN WHATEVER THEY WISH TO BELIEVE IN, WHETHER IT'S NOTHING OR THE FLYING SPAGHETTI MONSTER. PLEASE DO NOT ARGUE ABOUT ANYTHING.
I thought of something. Remember the scene in the Jaya arc where the Bellamy Pirates and the Straw Hat Pirates are in the bar, and they happen to mention they're searching for the One Piece and Sky Island?
The Bellamy Pirates couldn't stand that. They trampled over SH's dreams, hopes, and goals. They said Luffy was a fool for believing such a treasure and island exists. They start beating him, testing his strength to see how strong Luffy really is, since he believes in such a false thing.
Luffy, of course, does not fight back. He knows he can, he knows he can prove him wrong, he knows he can kill him, but he doesn't. He just stands there and takes it.
Now let's look at this at a real point of view.
Of course, everybody knows the one thing you must never discuss: religion. Because it all ends the same way. One disagrees, starts trampling on another's belief, calls them a fool, and tests them to see if they can argue back.
Out of browsing the internet, I notice the most common insults and attacks are from atheists. They call believers fools, saying they're wasting their time looking for something that "does not exist". Atheists test religious people to see if they can argue back, and when they don't, they keep calling them foolish. "They can't even argue back! Guess that's what happens when you believe in something with no evidence."
Of course, not all atheists are like this. It's actually anti-theists. The ones who attack religion and calls every part of it, and the people who follow it, foolish.
I'm not quite good at explaining this kind of thing, but I hope I get my point through. Maybe Oda created this scene to teach all the people who are being insulted for their beliefs not to fight back. They can mock, trample, hate, do all they can. Oda wanted to let people know "sometimes the best way to fight, is not to fight at all."
One Piece has taught many lessons in the past. I think this is one of them.
If you're being insulted for what you believe in, don't fight back.