The nativity: "an act of sovereign mercy"
"In David, Abraham, the Creation, the dominating epochs of the Old Testament are seized upon; the chronological nexus is, as it were, the exponent of the oneness of the divine work through the ages and of the divine purpose from the outset to lead up to the Messiah.
"The new procedure to be ushered in is throughout described as bearing a redemptive character. This is accomplished, first of all, by giving it, both in the objective announcement by God and in the subjective apprehension of those addressed, the background of a state of sin and unworthiness, and the corresponding signature of grace and salvation. God’s unique dealing with His people at this point is recognized as an act of sovereign mercy...There is no trace of the view that anything well-deserving has evoked this visitation of God, least of all anything resembling faithful observance of the law.
".... The source of all blessedness is sought in the berith (the covenant; crb), which is but another way of saying that it flows from the free promise of God. God fulfills what He promised to the fathers (the patriarchs) [Lk. 1.54, 55, 72, 73]." -- Geerhardus Vos, Biblical Theology, pp. 306,307