Ataba is a priest of Mau's religion. The best we can say about him? He sure is dedicated to his gods—so dedicated that he'll destroy evidence that they're not real.
Until the mutineers wash ashore, Ataba is Mau's primary source of conflict on the island. They argue a lot, way more than any adult man should yell at a teenage boy who isn't his son. Either Ataba's not a very good priest, or they don't have the golden rule where he comes from.
Now, we love a good debate, but Ataba crosses the line, calling Mau demon boy with practically every breath and just generally taunting him: "You believed! [...] Oh yes you did, just for a second! And you were fearful, and rightly so!" (5.103), he says to Mau after an unknown voice startles him. That's a great way to get people to believe: frighten them. (Hey, whatever works.)
Another reason the two are so bristly is that they're actually quite similar. Both adamantly refuse to bend their beliefs. Mau doesn't see himself in Ataba, though. When Ataba says he expects the gods to speak to him with an answer, Mau responds, "I think you're just hearing your own thoughts" (4.260). (The funny thing? he doesn't turn that observation inward when the Grandfathers are talking to him.)
To be fair, Ataba makes some good points. He calls Mau out on his alleged non-belief when he says, "you shake your fist at the sky and revile them for not existing! You need them to exist so that the flames of your denial will warm you in your self-righteousness!" (7.176). And his words are eerily prescient when he says "Do not dismiss the past so lightly, demon boy. It may still teach you something" (9.197).
Speaking of not taking his own advice, at one point Ataba tells Mau, "You want the truth to be a truth you like. You want it to be a pretty little truth that fits what you already believe!" (7.163). Note that this is before Ataba tries to destroy the newly found god stone that would invalidate his entire belief system. Now that's the pot calling the kettle black. Or, in island speak, the coconut calling the monkey hairy.
Ataba's inability to change his mind gets at this book's core opposition between science and religion. Science is about finding theories to match your evidence, and—importantly—changing your mind when you find new evidence. Ataba has the antithesis of a scientific mindset. He only wants evidence to match his theory—so much that he'll even destroy evidence that proves the contrary.
Mau's surprisingly nice about this, saying "I don't think he believes in his gods, but he believes in belief" (9.165). But this belief ends up being the death of him. Defending his religion (which has been proven false) Ataba is shot by a gun-wielding trouserman. Mau says, "[Ataba] was a good man [...] He deserved better gods" (11.143).