Pretty much every human culture on the planet has some form of religion. Many you've heard of Christianity? Islam? Hinduism? Judaism? Some you haven't. And some don't exist anymore. (Poor Aphrodite; no one really believes in her anymore.) A lot of these religions have elements in common: there are a lot of floods, a lot of sin, and a lot of gods.
Mau's religion isn't much different—except they weren't expecting the flood. Mau gets seriously ticked off at the gods after that whole catastrophe (understandably), and he rejects the whole thing. But by the end of the story, he's found out that the stories of gods aren't just crackpot fables. They have some kernel of truth that got twisted over the years, like a game of tribal telephone…or tribal coconuts-with-string. So what's a boy to believe in?