It is a sermon by Luther on Luke 2:10-11: The angel said to them, “Do not be afraid; for behold, I proclaim to you good news of great joy that will be for all the people. For today in the city of David a savior has been born for you who is Messiah and Lord.”
O, this is the great joy of which the angel speaks. This is the comfort and exceeding goodness of God that, if anyone believes, he can boast of the treasure that Mary is his rightful mother, Christ his brother, and God his father. For these things actually occurred and are true, but we must believe. This is the principal thing and the principal treasure in every Gospel. Christ must above all things become our own and we become his. This is what is meant by Isaiah 9:6: “Unto us a child is born, unto us a son is given.” To you is born and given this child.
Therefore see to it that you do not treat the Gospel only as history, for that is only transient; neither regard it only as example, for it is of no value without faith. Rather, see to it that you make this birth your own and that Christ be born in you. This will be the case if you believe, then you will repose in the lap of the virgin Mary and be her dear child. But you must exercise this faith and pray while you live; you cannot establish it too firmly. This is our foundation and inheritance, upon which good works must be built (Watch for the Light: Readings for Advent and Christmas 219-220)