How did Beowulf come to power in the book Beowulf?

Grendel breaks into Herot and starts killing soldiers, but when he approaches Beowulf, he is suprised as Beowulf spins around grabbing his arm, and Grendel realizes that Beowulf is blessed by God. Beowulf ends up tearing Grendel's arm off causing him to run away bleeding to death.