That which is conceived (or formed) in her - So I think γεννηθεν should be translated in this place: as it appears that the human nature of Jesus Christ was a real creation in the womb of the virgin, by the power of the Holy Spirit. The angel of the Lord mentioned here was probably the angel Gabriel, who, six months before, bad been sent to Zacharias and Elisabeth, to announce the birth of Christ's forerunner, John the Baptist. See Luke 1:36.
He thought on these things - He did not act hastily. He did not take the course which the law would have permitted him to do, if he had been hasty, violent, or unjust. It was a case deeply affecting his happiness, his character, and the reputation and character of his chosen companion. God will guide the thoughtful and the anxious. And when we have looked patiently at a perplexed subject, and know not what to do, then God, as in the case of Joseph, will interpose to lead us and direct our way. Psalm 25:9.
The angel of the Lord - The word “angel” literally means a messenger. It is applied chiefly in the Scriptures to those invisible holy beings who have not fallen into sin: who live in heaven (1 Timothy 5:21; compare Jude 1:6); and who are sent forth to minister to those who shall be heirs of salvation. See the Hebrews 1:13-14 notes, and Daniel 9:21 note. The word is sometimes applied to men, as messengers Luke 7:24; Luke 9:52; James 2:25; to the winds Psalm 104:4; to the pestilence Psalm 78:49; or to whatever is appointed to make known or to execute the will of God. It is commonly applied, however, to the unfallen, happy spirits that are in heaven, whose dignity and pleasure it is to do the will of God. Various ways were employed by them in making known the will of God, by dreams, visions, assuming a human appearance, etc.
In a dream - This was a common way of making known the will of God to the ancient prophets and people of God, Genesis 20:3; Genesis 30:1, Genesis 30:11, Genesis 30:24; Genesis 37:5; Genesis 41:1; 1 Kings 3:5; Daniel 7:1; Job 4:13-15; compare my notes at Isaiah. In what way it was ascertained that these dreams were from God cannot now be ascertained, It is sufficient for us to know that in this way many of the prophecies were communicated, and to remark that there is no evidence that we are to put reliance on our dreams. Dreams are wild, irregular movements of the mind when it is unshackled by reason, and it is mere superstition to suppose that God now makes known His will in this way.
Son of David - Descendant of David. See Matthew 1:1. The angel put him in mind of his relation to David perhaps to prepare him for the intelligence that Mary was to be the mother of the Messiah - the promised heir of David.
Fear not - Do not hesitate, or have any apprehensions about her virtue and purity. Do not fear that she will be unworthy of you, or will disgrace you.
To take unto thee Mary thy wife - To take her as thy wife; to recognize her as such, and to treat her as such.
For that which is conceived in her is of the Holy Ghost - Is the direct creation of divine power. A body was thus prepared pure and holy, and free from the corruption of sin, in order that he might be qualified for his great work the offering of a pure sacrifice to God. As this was necessary in order to the great work which he came to perform, Joseph is directed by an angel to receive her as pure and virtuous, and as every way worthy of his love. Compare the notes at Hebrews 10:5.