The story of Beowulf as the greatest Anglo-Saxon hero was passed down orally until it was written down in 700. It was originally only a pagan epic that got exaggerated and the story grew grander as time passed. Before the Christians took the story and changed it in their attempts to convert the Anglo-Saxons, the entire content of the story was violence and pagan heroic ideals such as revenge and dieing a noble death in order to make your life worth something. In the early tenth century a "new" Beowulf was circulated by the Christian missionaries in their attempts to communicate Christian teachings to the Anglo-Saxons in a way they would listen to and understand.
The first instance of Christian editorializing we see in Beowulf is in the chapter where he battles Grendel. Before he meets up with Grendel he has a small monologue where he presents himself as larger than life hero. He promises to make his fight with Grendel a fair one by not using a sword of shield. The last thing Beowulf says he will do is leave the outcome up to the infinite wisdom of God. He uses the singular noun there indicating he is not pagan. Then he goes on to say, "God will reward whom he chooses!" showing his great trust in fate.
Throughout the rest of Beowulf we see more Christian references. After Grendel's mother seeks out Herot and takes one of Hrothgars greatest friends. The passage that follows is one that totally goes against the entire Anglo-Saxon thinking and way of life. "They"d traded deaths Danes and Monsters, and no one had one both had lost!" To the Danes avenging friends" deaths was a great and honorable act. They lived in a very war-like culture where death was an every day reality. The Christians also changed Beowulf's death passage. He asks for a lighthouse to be built in the beach and for him to be put in it. This is a testament to his life and sacrifice. The lighthouse is a symbol of Jesus as a light in the darkness and a guiding force.