The Gospel according to St Matthew recounts the birth of Jesus from St Joseph’s viewpoint. He was betrothed to Mary who, “before they came together… was found to be with child of the Holy Spirit” (Mt 1:18). The Son of God, fulfilling an ancient prophecy (cf. Is 7:14), became man in the womb of a virgin and this mystery at the same time expressed the love, wisdom and power of God for mankind, wounded by sin. St Joseph is presented as “a just man” (Mt 1:19), faithful to God’s law and ready to do his will. For this reason he enters the mystery of the Incarnation after an Angel of the Lord appeared to him in a dream, announcing: “Joseph, son of David, do not fear to take Mary your wife, for that which is conceived in her is of the Holy Spirit; she will bear a son, and you shall call his name Jesus, for he will save his people from their sins” (Mt 1:20-21). Having given up the idea of divorcing Mary secretly, Joseph took her to himself because he then saw God’s work in her with his own eyes.
St Ambrose comments that “Joseph had the amiability and stature of a just man, to make his capacity as a witness worthier” (Exp. Ev. sec. Lucam II, 5: CCL 14,32-33). St Ambrose continues: “He could not have contaminated the temple of the Holy Spirit, the Mother of the Lord, the womb rendered fertile by the mystery” (ibid., II, 6: CCL 14,33). Although he had felt distressed, Joseph “did as the Angel of the Lord commanded him”, certain that he was doing the right thing. And in giving the name of “Jesus” to the Child who rules the entire universe, he placed himself among the throng of humble and faithful servants, similar to the Angels and Prophets, similar to the Martyrs and to the Apostles — as the ancient Eastern hymns sing. In witnessing to Mary’s virginity, to God’s gratuitous action and in safeguarding the Messiah’s earthly life St Joseph announces the miracle of the Lord. Therefore let us venerate the legal father of Jesus (cf. Catechism of the Catholic Church, n. 532), because the new man is outlined in him, who looks with trust and courage to the future. He does not follow his own plans but entrusts himself without reserve to the infinite mercy of the One who will fulfil the prophecies and open the time of salvation.
[Pope Benedict, Angelus 19 December 2010]
