What contrast is implied in the lines 80-102 of the Seafarer?
To me, the contrast that is implied is a contrast between the old, pagan, heroic values of the days gone by and the new, more Christian and peaceful values that the poet is recommending to people.
In the lines you mention, the poet seems to be talking about how the old days are gone and have been replaced by Christianity and a new way of thinking about what's good and honorable.
In the old days, there was nobility and glory and honor. But now we know that this glory and honor (and gold) is nothing in the face of God's power.
So this is saying, to me, that there is a contrast between the old days and the new and that the new days are better since they now know about the true God.