We are meant to notice a contrast between the figure mentioned at the outset of [Luke's infancy] narrative -- Caesar Augustus -- and the character who is at the center of the story. Caesar would have been the best-fed person in the world, able at a snap of his fingers to have all of his sensual desires met. But the true king, the true emperor of the world, is born in a cave outside of a forgotten town on the verge of Caesar's domain.
Caesar, we might say, represents the best possible outcome of the fall of Adam, the closest to a god that man can get on his own. And it's precisely at this moment in time, when God's chosen people are living the kind of life you get when there's someone who is the closest to a god that man can get on his own, that God gets the closest to man that God can get.