John Chrysostom — as excerpted in the Catena Aurea on Matthew 1:20

“Or, The Angel appeared to Joseph when he was in this perplexity, that his wisdom might be apparent to Joseph, and that this might be a proof to him of those things that he spoke. For when lie heard out of the mouth of the Angel those very things that he thought within himself, this was an undoubted proof, that he was a messenger from God, who alone knows the secrets of the heart. Also the account of the Evangelist is beyond suspicion, as he describes Joseph feeling all that a husband was likely to feel. The Virgin also by this was more removed from suspicion, in that her husband had felt jealousy, yet took her home, and kept her with him after her conception. 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. But to the Virgin the Angel announced her conception before it took place, lest if he should defer it till afterwards she should be in straits. And it behoved that Mother who was to receive the Maker of all things to be kept free from all trouble. Not only does the Angel vindicate the Virgin from all impurity, but shews that the conception was supernatural, not removing his fears only, but adding matter of joy; saying, That which is born in her is of the Holy Spirit.”