Matthew 1:20

But while he thought on these things
While he was revolving them in his mind, considering what was most fit and proper to be done, whether to dismiss her publicly or privately; while he was consulting within himself the glory of God, the peace of his own conscience, and the credit of Mary,

behold the Angel of the Lord appeared to him in a dream;
probably the same Angel which appeared to Zacharias, and brought him tidings that his wife should have a son, and who also appeared to Mary, and acquainted her that she should conceive, and bring forth the Messiah, Whose name was Gabriel, ( Luke 1:11 Luke 1:19 Luke 1:36 ) . If we will believe the Jews, this Angel must be Gabriel, since he is the Angel who they say ^F4 (amlx le anmmd) "is appointed over dreams"; for he appeared to

Joseph in a dream,
which is one of the ways and methods in which the Lord, or an Angel of his, has appeared to the saints formerly, and has answered them, see ( Genesis 31:11 ) ( 1 Kings 3:5 ) ( 1 Samuel 28:6 1 Samuel 28:15 ) and is reckoned by the Jews ^F5 one of the degrees or kinds of prophecy: and so the Angel here not only encourages Joseph to take to him his wife,

saying Joseph, thou son of David;
which is said partly to attest his being of the house and lineage of David, and partly to raise his expectations and confirm his faith, that his wife should bring forth the promised son of David; and chiefly to engage his attention to what he was about to say,

fear not to take unto thee Mary thy wife;
do not be afraid either that thou shalt offend the Lord, or bring any reproach or scandal upon thyself as if thou didst connive at an adulteress; but as she is thine espoused wife, solemnly betrothed to thee, take her home to thyself, live with her as thy wife, and openly avow her as such. To which he is encouraged by the following reason or argument,

for that which is conceived in her is of the Holy Ghost;
she has not been guilty of any criminal conversation with men; this conception of her's is of the Holy Ghost, and entirely owing to his coming upon her, and overshadowing her in a wonderful and miraculous manner. I say, the Angel not only encourages Joseph after this manner, but delivers something to him by way of prophecy, in the following verse.