I have to wonder just how many of the explicitly monotheistic references in the various translations were added in after the fact. The original poem has been translated so many times and by so many authors -- each of whom had personal ideas about religion -- that I treat explicitly "Christian" references with skepticism. I don't doubt that religion played a part in the tale, but to interpret it through a single lens, fitting lines to a single doctrine of religion, seems disingenuous to me. However, there is little doubt that Beowulf is a religious character; it's just the specific flavor of religion that I question.
I would agree that the Christian influences in the poem suggest an afterlife, but there isn't much direct comment on the quality of the afterlife. There are no characters who say that they will have it easier in the afterlife once they die and are relieved of the burdens of earthly life. That was definitely a belief of early Christians, but that is not a big topic of comment in the text of the epic.
It is important to remember that Beowulf is a text that seems to acknowledge the passing of primeval animism in Britain and its replacement with Christianity. Christianity as a religion permeates the text and is something that is impossible to ignore, and this is something that the posts above have given you many examples of. As a result of this, I think there is a definite acknowledgement of a Christian afterlife in this text, as it is so profoundly influenced by Christianity.
Beowulf offers much praise to God for allowing him to succeed in his battles. One could justify that he does this in order to insure his place in the after-life. If he were not concerned with the after-life, he would not have made references to God (as seen in the following):
Let the wise God, the holy Lord, decree success on whichever side seems right to Him!”
Early in the poem, the great Danish king Shield Sheafson dies. The narrator comments (in the Seamus Heaney translation) that
Shield was thriving when his time came
and he crossed over into the Lord’s keeping. (26-27)
Near the end of the poem, Beowulf dies and the narrator comments that his
. . . soul fled from his breast
to its destined place among the steadfast ones. (2819-20)
Both of these quotations are just two of the many passages in the poem that imply the existence of a Christian afterlife.
On the one hand, there is obviously a heavy emphasis in Beowulf on one's deeds during life. These are the things that earn warriors everlasting fame. On the other, the title character makes frequent reference to God's will, and at times seems to conflate the concept with a pagan concept of "fate." The examples above are good ones as regards his rejection of riches, but ultimately I think the tension between Germanic pagan values and Christian ideals is basically unresolved.
People love to believe in heroes. I think that the reason Beowulf is inspiring is because it is a fun tale of heroism. People like drama. They like facing the hero against impossible odds, and watching the hero defeat the monster.
I know that the original tale of Beowulf was told before it was copied down with Christian references, so that as pohnpei397 notes, the idea (as with the Egyptian Pharaohs) that one might needs his treasures after death might promote the idea of an afterlife.
However, the version of Beowulf that has survived has clear references to Cain the murderer, and Grendel's descent from the line of Cain (showing a Christian influence). We could also infer that Beowulf is buried with the treasure to remind him in his last moments of the things he achieved in his life time that he would be leaving behind.
At the end of the story, it says:
Yet it was not greed for gold, but heaven's grace that the king had ever kept in view.
This shows that Beowulf the warrior cared more about an afterlife in heaven than for treasure.
He also thanks God for enabling him to win the treasure for his people:
I give thanks to my God, to the Wielder-of-Wonders, for the gold and treasure upon which I now gaze; I thank Heaven's Lord that I have been given grace to acquire such for my people ere the day of my death has come! Now have I bartered the last of my life for the hoard of treasure, so look well to the needs of my land!
His wishes are to care for his people before he dies. The fact that he speaks at such length about God's blessings for his people—through him—makes me believe that he will not embrace simply the sense of God's blessings, but that as he dies he will also have the hope of a life after death. It doesn't make sense that the text influenced by Christianity would leave that detail out.
I suppose that you could argue that the fact that Beowulf has the treasure buried with him, rather than having it distributed to the people, can be seen in this way. It implies that treasure is not what is valuable in life. That would imply that the next life would be less materialistic.