But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, son of David, fear not to take unto you Mary your wife: for that which is conceived in her is of the Holy Spirit.
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How then did the angel assure Joseph? Hear and marvel at the wisdom of these words: “Joseph, son of David, do not fear to take Mary your wife.” The angel instantly puts him in mind of David, from whose seed the Anointed One would spring. He did not allow him to be confused by the exalted title of his forefather or remind him that the promise was made to the whole race. Rather, he addresses him personally as “Joseph, Son of David.” … By saying “fear not,” he indicates that Joseph had been afraid, lest he might give offense to God by retaining an adulteress under the law. If it had not been for this, he would not have even thought of casting her out. The angel came from God to bring forward and set before him clearly what he thought to do and what he felt in his mind. The angel did not only mention her by name but also simply called her “your wife.” He would not have called her so if she had been unfaithful. Even as espoused, he speaks of her as “your wife,” just as Scripture commonly calls betrothed husbands sonsinlaw even before marriage. But what is meant by “[Do not fear] to take Mary your wife”? It means to retain her in his house. For he was intending to put her away. It is as if the angelic voice prompted: “Retain her just as if she has been committed to you by God, not by her parents alone. God is committing her not for marriage but to dwell with you. By my voice he is committing her to you.” Just as Christ would later commit Mary to his disciple, so now he commits her to Joseph. The Gospel of Matthew, Homily