From: Christmas as the End of History
And I think the analogy of the river helps us see how.
Picture the river as redemptive history flowing toward the ocean which is the final kingdom of God, full of glory and righteousness and peace. At the end of the river the ocean presses up into the river with its salt water. Therefore, at the mouth of the river there is a mingling of fresh water and salt water. One might say that the kingdom of God has pressed its way back up into the river of time a short way. It has surprised the travelers and taken them off guard. They can smell the salt water. They can taste the salt water. The sea gulls circle the deck. The end has come upon them.
Christmas is not another bend in the river. It is the arrival of the salt water of the kingdom of God which has backed up into the river of history. With the coming of Christmas, the ocean of the age to come has reached backward up the stream of history to welcome us, to wake us up to what is coming, to lure us on into the deep."