Concerning "The Seafarer," Christianity came to what we now call Great Britain in the sixth century A.D, brought by missionaries from continental Europe, for the most part. Celtic and Anglo-Saxon beliefs would have been popular before Christians arrived, and well after.
The poem, I don't think, really deals with how "religion" arrives in Britain. Specifically, it doesn't deal with how Christianity arrives. The poem is a mixture of non-Christian and Christian thought. Fate, for instance, is an idea possibly inherited from the Greeks, who were obviously non-Christian. The poem doesn't specifically deal with how religion got there, though.
Thus, I'm not sure exactly what your instructor is looking for, if the question is worded as you've worded it.
Christian thought in the poem is something to be studied, and something that can shed light on Christianity in Britain, though not the history of how it arrived.
The didactic, sermon-like passages in the poem (lines 64-80 and 103-124) are possibly interpolations. "The Seafarer" is likely an oral work that was eventually written down by a Christian monk. No copyright laws existed then, and the concept of personal ownership of a work of writing was much different then than it is today. It's possible that the monk who wrote the poem down added his "two cents worth," as they say. The theory is that the monk writes down the Anglo-Saxon, non-Christian work, and while doing so attempts to Christianize it.
The passages suspected of being interpolations seem to some commentators to lack unity with and relevance to the rest of the work, and to be different in tone and purpose.
For a specific example, the "Thus," in line 64 suggests to me that the lines are something added by a monk, since thus is like a therefore, and should be the culmination of what comes before it. Before this thus is lots of misery and swearing at the ocean, yet the thus leads to "the joys of God" and great praise for God, etc. The suspected passages are extremely didactic, or sermon like.
Thus, the poem, though it doesn't deal directly with how religion arrived, may share light on Christianity once it arrived.