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.
All Commentaries on Matthew 1:20 Go To Matthew 1
Theophylact of Ochrid
When the righteous one was uncertain, then the angel appeared in order to show him what to do. The angel appeared to him in a dream, because Joseph had great faith. With the shepherds the angel conversed openly, as they were rough country men; but the angel spoke to Joseph, who was righteous and believing, in his sleep. How could Joseph not believe, when the angel spoke to him of matters that were in his mind and that he had not revealed to anyone? For it says, "while he pondered," but did not speak of, these things, the angel appeared to him. It was right that he believed the angel to be of God, for it is the attribute of God to know the things that are unspoken.
The angel called him "son of David" to remind him that the prophets had foretold that the Christ would come from the seed of David. It is as if the angel were saying to him, "Do not doubt, but remember David who received the promise concerning Christ."
Here he shows that Joseph was afraid to keep her, lest he spurn God by harboring an adulteress. Or, in another sense, "Fear not," that is, "Though you fear to touch her who has conceived of the Holy Spirit, do not fear to take her unto thee, that is, to keep her within your house." For in his thoughts and deliberations he had already divorced her.
This means, you perhaps think that she is an adulteress. But I say to you that she is your wife, that is, she has not been corrupted by anyone, but she is your own betrothed.
Not only is she acquitted of any unlawful union, but she has conceived in some divine and wondrous manner. Therefore you ought rather to rejoice because of this.