Is there a tension between the values of these warrior peoples and the suggested Christianity of the poet in the poem Beowulf?

There may be a tension between New Testament Christianity as we understand it and the values of the Dark Age Danes portrayed in Beowulf.  But the brand of Christianity apparently held by the poet seems to fit very well with the demands of his warrior culture.
When thinking of Christianity, most modern readers think first of Jesus and His New Testament teaching, especially the Sermon on the Mount. (See Matthew 5:1 – 7:27.)  It is this sermon which contains the most famous sayings of Jesus, and also those which sound the most pacifistic, such as “Blessed are the meek,” “Turn the other cheek,” and “If someone wants to take your tunic, let him have your cloak as well.”  Taken in isolation, these sayings would seem to be urging the exact opposite of the lifestyle led by Beowulf and company, whose core values seem to be bravery, boastfulness, and vengeance.
But actually, this sharp contrast depends on caricatures of both the epic of Beowulf and of Jesus’ teaching.
The warriors in Beowulf are indeed tough, violent, and not friends of mercy.  However, this does not mean they have no moral code.  They value loyalty to their king or leader, loyalty to their family and clan (sometimes expressed in vengeance), generosity (the king gives gifts), and self-sacrifice (Beowulf’s exploits are done not only to prove himself but to protect the weak who are being slaughtered by Grendel and the other monsters).   Though violent, they are not just barbarians living in chaos.
Jesus’ teaching is also much more rich and complex than “let people walk all over you.”  If we look at the totality of his teaching, He preaches a way of life that calls for a great deal of loyalty, generosity, and courage to carry it out.  And though, in the Sermon on the Mount, He was warning us against being contemptuous, selfish, and greedy, He does not ever say that we may never defend ourselves or go to war.  For example, in Luke 3:10 – 14, John the Baptist, who is calling the people to “repent” and prepare for Jesus’ coming, does not tell Roman soldiers or even tax collectors to leave their jobs; He tells them to do their jobs in a fair way.  This is also the tenor of Jesus’ teaching.
As for the poet’s understanding of Christianity, he does not mention Christ's teachings.  He has clearly heard of the God of the Bible.  He is familiar with certain (not all?) Old Testament events; for example, he says that Grendel and other monsters are descended from Cain. 
The poet’s understanding of God actually elevates his warrior values to a beautiful philosophy.  In an extended discussion, the poet points out that God alone knows, and chooses, when it is a man’s time to die.  This should make a man brave in battle, and it should make him die quietly and uncomplaining, knowing that he cannot stop his death.  Though the poet does not use the word, he is articulating the theological doctrine of God’s sovereignty.  This is indeed a doctrine that can give people peace in the face of terrible events and can help to make us calm and brave, if we believe it.   For example, in Revelation 1:17 – 18, Jesus says, “Do not be afraid … I hold the keys of Death and Hades.”  
Both in Christianity and in the values of Beowulf, death is not the worst thing that can happen to a person.
This idea of God being over all also influences how kings are viewed in Beowulf.  Again, we are not dealing with barbarians where the only law is the king’s whim.  Rather, the king is also governed by certain values.  Though there are good and bad kings, and bad kings are not immediately deposed, there is a sense of a value system that is higher than any individual king, and by which kings can be judged.
All in all, the “Christianity” or theology of the Beowulf poet fits in surprisingly well with his warrior culture, and seems to strengthen all that is best in it.