What is the possible irony underlying the final lines of the prologue in beowulf?

My modern translation has:

Grave were their spirits,
mournful their mood. No man is able
to say in sooth, no son of the halls,
no hero 'neath heaven, -- who harbored that freight!

Although Beowulf was a Christian, he was given a pagan funeral. The line asks if anyone really knew what Beowulf's afterlife would be. If Beowulf died a heroic Christian warrior, he would certainly go to Heaven.