This question dominated early-twentieth-century literary and folklore studies. Classic works by Raymond Wilson Chambers, Beowulf: An Introduction to the Study of the Poem, and F. W. Panzer, Beowulf, provided a bedrock that identified both pagan and Christian elements. The core of the story is clearly pre-conversion: the story is part of the widespread "bear's son" complex of stories catalogued by Aarne and Thompson as Tale Type 301. (Beowulf is the "bee-wulf", i.e. the ravager of bees, a euphemism for a bear). The story appears in diverse early sources including the Icelandic Saga of Grettir the Strong.

Widespread, early examples of the story in medieval literature indicates that much of this story resided in oral tradition before the point of conversion. That said, there is no question that a Christian author recorded the poem and may have influenced some details and composition. What survives is a hybrid, but as your question asks specifically about what is "at the heart" of this poem, the answer to that must be a pre-conversion oral tradition.

If you look to early Christian missionaries/Christianization in Europe, there is a common theme of cultural appropriation as an evangelistic tool. The idea is this: "Yes, your legends and religions and principles are 100% true, but XYZ is actually Jesus and ABC is the Christian God, etc."

This comes from a concept called Illuminationism (which was something St. Augustine asserted, around the same time much of Europe was being Christianized): humans know the essences of these truths (ie they tell true stories such as Beowulf), but God illuminates these truths for what they really are (ie Christian truths). Many early missionaries (eg St Patrick) put this idea into practice with evangelizing in their respective areas. "All truth is God's truth" is a quote that is commonly attributed to Augustine, but I am not sure if that is well sourced or not.

Now, on the flip side, you do have a type of hybridization coming into Christianity from these cultures as well. Mithraism and other Mystery cults are good examples of this. Here, an element like the Halo can work it's way into depictions of Jesus, the saints, etc. There is also an element of competition that arises with Christianity vs. the local religions, and I think the success of Christianity vs. these other cults has a lot to do with Christians finding it easy enough to claim certain concepts, images, icons, and oral/written histories/traditions as part of a greater narrative of what Christ is doing on Earth.

Please note that when using the word "cults" in this context has nothing to do with what are known as "cults" today. Research in the history of religion uses a work like "cults" to denote a smallish, newish religious group. The word is pretty loaded today, so a lot of contemporary research is referring to them as "new religious movements". Cults in a historical context is a lot less loaded. I had a professor (Dr. Tina Sessa, if anyone is familiar, tons of great work, and she told me I was "impressive" once and my big paper for her was "brilliant", so I like her) who made a great argument that early Christianity was a run-of-the-mill apocalyptic-prophecy cult until Constantine started building churches.