A.T. Robertson Commentary Matthew 1:20

"But when he thought on these things, behold, an 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 Spirit." — Matthew 1:20 (ASV)

An angel of the Lord appeared unto him in a dream (αγγελος κυριου κατ' οναρ εφανη αυτω). This expression (αγγελος κυριου) is without the article in the New Testament except when, as in 1:24, there is reference to the angel previously mentioned. Sometimes in the Old Testament Jehovah Himself is represented by this phrase. Surely Joseph needed God's help if ever man did. If Jesus was really God's Son, Joseph was entitled to know this supreme fact that he might be just to both Mary and her Child. It was in a dream, but the message was distinct and decisive for Joseph. He is called "Son of David" as had been shown by Matthew in Mt 1:16. Mary is called his "wife" (την γυναικα σου). He is told "not to become afraid" (ingressive first aorist passive subjunctive in prohibition, (μη φοβηθηις), "to take to his side" (παραλαβειν, ingressive aorist active infinitive) her whom he had planned (ενθυμηθεντος, genitive absolute again, from εν and θυμος) to send away with a writ of divorce. He had pondered and had planned as best he knew, but now God had called a halt and he had to decide whether he was willing to shelter Mary by marrying her and, if necessary, take upon himself whatever stigma might attach to her. Joseph was told that the child was begotten of the Holy Spirit and thus that Mary was innocent of any sin. But who would believe it now if he told it of her? Mary knew the truth and had not told him because she could not expect him to believe it.