Albert Barnes on Matthew 1:20

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, Ps 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, (1Timm 5:21, comp. Jude 1:6) and who are sent forth to minister to those who shall be heirs of salvation, Heb 1:13, 14, Dan 9:21. The word is sometimes applied to men, as messengers, (Lk 7:24, 9:52, Jas 2:25) to the winds, (Ps 104:4); to the pestilence, (Ps 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 only 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, Gen 20:3, 31:10,11,24, 37:5; Gen 41:1, 1Kgs 3:5, Dan 7:1, Job 4:13-16. In what way it was ascertained that these dreams were from God, cannot now be told. It is sufficient for us to know that in this way many of the prophecies were communicated; and to remark, that now 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. Is 37:1.

Son of David. Descendant of David. See Mt 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 fears about her virtue and purity. Do not fear that she will be unworthy of you, or will disgrace you.

(w) "in a dream" Mt 1:16 (1) "conceived" or, "begotten"