20 But while he thought on these things, behold the angel of the Lord appeared to him in his sleep, saying: Joseph, son of David, fear not to take unto thee Mary thy wife, for that which is conceived in her, is of the Holy Ghost.
Pseudo-Chrysostom: By addressing him as son of David, he sought to recall to his memory the promise of God to David, that of his seed should Christ be born.
By the words, “Fear not,” he desired to shew that he knew the heart; that by this he might have the more faith in those good things to come, which he was about to speak concerning Christ.
There were three reasons why the Angel appeared to Joseph with this message. First, that a just man might not be led into an unjust action, with just intentions. Secondly, for the honour of the mother herself, for had she been put away, she could not have been free from evil suspicion among the unbelievers. Thirdly, that Joseph, understanding the holy conception, might keep himself from her with more care than before.
Chrysostom: By saying, “Be not afraid,” he shews him to be in fear that he had offended God, by having an adulteress; for only as such would he have ever thought of putting her away.
The account of the Evangelist is beyond suspicion, as he describes Joseph feeling all that a husband was likely to feel.
She had not told Joseph the things that the Angel had said to her, because she did not suppose that she should be believed by her husband, especially as he had begun to have suspicions concerning her.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. I (London: Rivington, 1842).