I'd like to add something on the Christian elements of Beowulf. Obviously, whoever wrote down the old oral legend was a Christian monk, but why would he expend his energies in recording an obviously pagan story when there were so many classics of Christian literature to be preserved? I think it's quite probable that he saw a reflection of his own spiritual war in the story of Beowulf.
For those who didn't go to Catholic school, a bit of explanation is in order. A Christian monk took a threefold vow; he promised poverty, chastity, and obedience as a way of combatting the three main sources of sin:
The World was all the temptations to power over one's fellow man, amongst them glory, riches, and temporal power. Grendel, a force of pure malignant greed and violence, is a fairly good representation of the man who lets the World rule his life, and Beowulf's triumph over him represents the monk's faithfulness to his vow.
The Flesh represents not only sexual desire, but also the ties of kinship. A reading of Beowulf will give one some inkling of the blood feuds, based entirely on ties of family, that tore apart the society of the times. Grendel's Mother engages in this kind of revenge killing, and Beowulf's beheading her could be seen as symbolizing the monk's forsaking of children and kin.
The Devil is the most obvious tie of Beowulf's struggles to those of the monk. From ancient times, the Dragon has symbolized pure malevolent evil to Westerners and as such often represents the Devil. Beowulf tries to combat the Dragon alone and ultimately fails. Here the importance of the monk's third vow becomes apparent; no man could fight the supreme evil being alone, and obedience to God and one's superiors was necessary as discipline for a spiritual army.
The above is compiled from long debates in high school English class and given my interpretation.