Why is Grendel so disgusted by the speeches of the Fourth Priest?

The fourth priest that comes out is actually younger and more vibrant than the other priests. He talks about how great it is that Ork, so rational a priest, is thinking outside the box. He thinks Ork's revelation about the Destroyer is true and revels in the thought of meeting him. Grendel intended to slaughter the priests before the fourth priest showed up. Because he liked the fourth priest's speech on religion, he merely slinks away.