The angel Gabriel announced to Mary that she would bear a son and call his name Jesus and that her son, the son of King David and also the Son of God, would reign forever (Luke 1:26-38).

Consider what Reformed theologian Karl Barth has written about Gabriel's announcement of God's mercy and Mary's song of praise:

Before the beginning of the true Gospel account of the life and suffering and death of Jesus, the angel of God under the name of Gabriel plays an emphatic role with his twofold message, first to Zacharias and then to Mary. . . .

The ministry of the angel of the Lord of Luke 1 is to announce this coming event, which is twofold and yet one, since the first is only the preparation for the second. The angel himself is only the preceding shadow or sound of this event. He is only the herald of the God who will come in person in this event.

As in the early accounts of the history of the covenant, there again appears the heaven which comes to earth with God Himself, epitomised in the form of the one angel of the one God. That he is really a heavenly creature may be seen from the first effect of his appearance on both those to whom he comes. Zacharias and Mary are both said to be "troubled." And they are both told not to fear. It is not the fact that he is a heavenly creature, and therefore strange to earthly beings, which makes the angel an angel. It is his message, the Word of God which he has to deliver in all his strangeness as a heavenly creature.

And for both the persons concerned, and therefore in relation both to the coming precursor and the coming Christ Himself, this message obviously has the character of an annunciation of the covenant of grace which is not merely promised again but is now fulfilled. . . .

And to this there corresponds equally clearly, when the announced event begins to come to pass, and has already done so in part, the unequivocally positive, grateful and joyful tenor of the songs of praise both of Mary and Zacharias. These two canticles form the climax of the chapter, and if we are rightly to understand the advent-angel of the chapter we do well to begin by considering them.

What the ministry of the angel accomplishes according to this story, and what is obviously therefore the meaning and purpose of his mission, is that the human creation to which he announces the coming of the kingdom, its King and its last and first witness, is thereby caused to break out in praise of the One who has willed this and begun to accomplish it; into praise of his mercy (note that the word occurs in both songs) as it is revealed and operative in this action.

Karl Barth, Church Dogmatics, edited by G. W. Bromiley and T. F. Torrance, III/3 (Edinburgh: T. & T. Clark, 1957), pp. 503-504, emphasis added.

So may we who hear the announcement yet today "break out in praise of the One who has willed this and begun to accomplish it."