Matteo 1:20

It was here that God interfered in behalf of the mother of His Son, according to His humanity: But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying: Joseph, thou son of David, fear not to take unto thee Mary, thy wife; for that which is conceived in her is of the Holy Ghost.

Joseph's mind was still busily engaged with the perplexing problem; he was wrestling with painful, distressing, distracting thoughts, and even his kind expedient may have seemed harsh to him. But, behold! a vivid introduction of the angelic appearance to emphasize the intervention of God. In a dream the vision came to Joseph to save him and his betrothed from an act which would result in disastrous consequences.

The appearance of an angel in a dream was one of the methods which God used to "make known His will, or to reveal the future in special cases. The angel addresses Joseph, "Thou son of David," not to awaken the heroic mood, as has been suggested, but to emphasize the thought of the legal acknowledgment and adoption of the child. He should not fear to take home, publicly to accept, Mary as his wife.

This simple acceptance of the angel's words meant for Joseph an act of faith similar to those performed by the great heroes of the Old Testament, to believe the Lord absolutely, in spite of all the evidences of the senses. This public recognition would save the honor of Mary and also that of her child. For instead of being the fruit of adulterous and licentious intercourse, the product of a most unholy cohabitation, the child which was to be born of her was of the Holy Ghost, begotten by deliberate intervention of God, against the course of nature.