Beowulf is generally understood to have been first composed during a pagan era in Anglo-Saxon culture, with the poem being updated later to tether it to the new Anglo-Saxon context of the early medieval period, when it was committed to paper (or, in this case, vellum). Accordingly, we see a struggle between paganism and Christianity in the poem which is at times uncomfortable, a situation we see in many Anglo-Saxon works. In "The Wanderer," for example, a clearly pagan elegy about isolation and the loss of vassalage is tied uncomfortably and abruptly in the conclusion to Christian themes of loyalty to God. In "The Dream of the Rood," this theme of marrying heroic concepts to Christianity reaches its peak in the depiction of Christ as the heroic king of the poem, in true Anglo-Saxon fashion.
In Beowulf, then, we have a poem where the pre-Christian Anglo-Saxon preoccupations form the backbone but where Christian concepts have evidently been inserted later. Beowulf, as a king, is every inch the Anglo-Saxon heroic ideal. He is a warrior with all the trappings: a named sword, super strength, the capacity to hold his breath under water for a superhuman length of time, and so on. The warriors in the poem enjoy Anglo-Saxon ideals of what makes a successful life: they are together in a mead hall, while Grendel is bitter and dejected at being excluded from this kind of pagan warrior unity. Although Grendel is described as the descendant of Cain, a Biblical tether, the passage describing the way he prowls around Heorot in envy of the soldiers is a pure Anglo-Saxon concept: the isolated man who has somehow transgressed against his society, wishing he could return to it. We see this concept in "The Wanderer" and "The Seafarer," and it was a key preoccupation of pre-Christian Anglo-Saxon society. While the author of this poem may have been a Christian, making his depiction of pagan Anglo-Saxon culture slightly inaccurate, the life-cycle of the Danes in the poem is pagan even up to the very point of Beowulf's death, where he receives a decidedly pagan burial surrounded by his treasures. Beowulf is a man whose life is governed by the Anglo-Saxon idea of 'wyrd' or fate, and when he is laid to rest, his treasure and all he has achieved in this life goes with him--"eldum swá unnyt swá hyt aérer wæs" (as useless to men now as it ever was).
By contrast to these deeply-rooted pagan elements in the poem, the Christian elements are rather superficial, clearly afterthoughts. We can see this, for example, in lines 100-115 when Grendel is described in relation to God in curiously un-Biblical terms--the poem says that "eotenas ond ylfe ond orcnease/swylce gigantas, tha with gode wunnon lange thrage"--"ogres and elves, and spirits from the underworld; also giants...strove with God for an interminable season." Even as the poem attempts to marry these monsters to the new Christian context, they remain distinctly rooted in Anglo-Saxon paganism. Another example of a Christian extension to an existing passage comes in lines 179-188, which talks about "the hope of heathens"--the poet here reflects that the heathens in the poem "ne wiston hie drighten god"--they "were not aware of the Lord God/nor yet the Helm of the Heavens." The poet goes on to lament that all those who do not know God will go to hell, while those who seek the Lord will be saved. This section is quite at odds with what comes before it and is generally believed to be one of the most obvious later additions to the poem.