I'm guessing you're speaking of Halo. During Halo 2, the Prophets promoted the Brutes to highest level of the Covenant, because they believed the Elites didn't truly believe in the Great Journey. The Elites then got pissed, because the truth of the Great Journey was revealed to the Arbiter. So he lead the Elites to go against the Brutes and Prophets, knowing that if they joined with the humans, they can stop the Prophets from activating the rings.
^^ - El Diablo122