It is strange that the gospel read at the beginning of the time of preparation for Christmas is that of the end of the whole history of the world. Yet that is not really surprising. For what is afoot in a small beginning is best recognized by the magnitude of its end. What was really meant and actually happened by the coming, the "advent," of the redeemer is best gathered from that completion of his coming which we rather misleadingly call the "second coming." For in reality it is the fulfillment of his one coming which is still in progress at the present time.
Karl Rahner, Everyday Faith, 1968, quoted in An Advent Sourcebook, Thomas J. O'Gorman, editor, page 158