In what way could the battle between grendel and beowulf be seen as an allegory for the battle between the pagans and christianity?

Grendel is presented in the Beowulf story as an embodiment of ungodly evil, and so in the defeat of Grendel by Beowulf can be seen as an allegory for the battle between good and evil and between Christianity (which was then taking root in England) and paganism.

Beowulf is an old, old story by an unknown Anglo-Saxon poet, written in Old English.  It stems back to around 1000 AD.  By that time, England had become largely Christianized, and so the cultural context of the epic poem would naturally include allusions to Christianity overcoming paganism.  In the story, Grendel and his mother are called "descendants of Cain," a reference to the biblical figure of the first son of Adam and Eve, Cain, who became the world's first murderer and a figure associated with evil and chaos and abandonment of the true God.  Beowulf can be seen as something of a "Savior" to defend what is right and good.

Frederick Klaeberg, in his analysis, Beowulf and the Fight at Finnsburg (1950),  noted that we might recognize features of the Christian Savior, Jesus, in Beowulf, who is depicted as "the destroyer of hellish fiends, the warrior brave and gentle, blameless in thought and deed, the king that dies for his people."

the influence of charles louis de secondant , baton montesquieu.