Is Beowulf originally a Viking story told by Christians & changed or a Christian story told to teach the Vikings about Christianity?
Although scholars have for three centuries argued relentlessly about the composition of Beowulf, and continue that argument today, most have put the poem's composition between 600 to 900AD, with most believing that the poem was most likely composed between 700-800AD. The consensus among Beowulf scholars is that the poem was composed by a monk, but the location of the monastery is still completely debatable--almost everyone who studies the manuscript concludes that many monasteries throughout England were sufficiently capable of producing the author. One leading scholar has suggested, for example, based on linguistic evidence, that the poem was composed in or near Mercia and has even argued that the author could be St. Aldhelm. Without new evidence, we shall never know, and the Beowulf poet will always be known as "Anonymous."
Christianity in England was, by 600, well established, and we have internal evidence in Beowulf that the poem's audience was completely conversant with biblical references. When the poet introduces Grendel, he provides Grendel's heritage:
With fulsome monsters/this sorrowful man had stayed awhile,/since the Shaper had condemned him/as Cain's kinsman. The Captain Eternal/had avenged the murder when Cain slew Abel;/that feud brought no joy . . . /
The use of "Shaper" and "Captain Eternal" rather than to Fate or Destiny place these lines in a Christian ethos. More important, however, is the poet's reference to Grendel as a kinsman of Cain and the allusion to the biblical story of Cain's murder of his brother Abel. If the poet had explained the Cain-Abel story, then we might conclude that the audience was, if not pagan, at least new to Christianity. The lack of an explanation leads us to believe that the audience was completely aware of the major events of the Old Testament and required no further explanation from the poet. The poet discusses the Cain-Abel connection here and when introducing Grendel's mother.
Because the poet waivers at times between Christian and pagan references--the poet shifts from God to Fate or Destiny in the same line--we are confronted with a poem that seems to reflect both Christian and pagan worlds. After all, the dragon who kills Beowulf is not a Christian construct but comes directly from Norse mythology, as does the entire narrative--Beowulf, the good leader, earning his reputation by killing a monster and ending his life protecting his kingdom from another monster. Sometimes, the Christian references actually seem out of place, but that is because the narrative has its roots in Scandinavia in a pre-Christian era.
One of the poet's main subjects--setting aside the Beowulf narrative--is the dynastic struggle between the Danish Scyldings and the Geats of Hrethel's kingdom and heritage. Much of Beowulf is actually about the various wars and feuds, including family disputes, among several Scandinavian tribes, most of which takes place in a pre-Christian era. In these sections, even though the poet injects Christian references, he often uses pagan terms most likely because he is re-telling a pagan tale.
In sum, then, it is reasonable to conclude that Beowulf is a poem composed by a Christian and addressed to a fully Christian audience, but re-telling that audience a version of their pagan heritage.