what about the religious act causes grendel angst?

In the woods Ork, an old priest, approaches Grendel thinking that he is a deity called the "destroyer" Grendel is both fascinated, moved and perplexed by this. Three other priests come in and admonish Ork for speaking with Grendel. Grendel runs away. He feels a strange dread about religion, monotheism, and these priests.