How does the poet use Christianity to show Grendel's nature?

The poet employs Christian themes in "Beowulf" to illustrate Grendel's nature as inherently evil and monstrous. Grendel is depicted as a descendant of Cain, the biblical figure who committed the first murder, which underscores his role as a symbol of sin and malice. This connection to Cain emphasizes Grendel's isolation from humanity and his perpetual state of conflict with the divine order, reinforcing the idea that he is a creature of darkness in opposition to the light of Christianity. Thus, the use of Christian imagery serves to highlight Grendel's malevolent essence and his tragic fate as a being cursed by God.