Now the birth of Jesus Christ happened this way: When his mother Mary was espoused to Joseph, before they came together, she was found to be with child of the Holy Spirit.
All Commentaries on Matthew 1:18 Go To Matthew 1
Augustine of Hippo
De Nupt. et Concup., i, 12: There was no carnal knowledge in this wedlock, because in sinful flesh this could not be without carnal desire which came of sin, and which He would be without, who was to be without sin; and that hence He might teach us that all flesh which is born of sexual union is sinful flesh, seeing that Flesh alone was without sin, which was not so born.
in App. 122 et. al.: Christ was also born of a pure virgin, because it was not holy that virtue should be born of pleasure, chastity of self-indulgence,incorruption of corruption. Nor could He come from heaven but after some new manner, who came to destroy the ancient empire of death. Therefore she received the crown of virginity who bare the King of chastity. Farther, our Lord sought out for Himself a virgin abode, wherein to be received, that He might show us that God ought to be borne in a chaste body.
Serm. 236 in App.: But not, as some impiously think, are we to suppose, that the Holy Spirit was as seed, but we say that He wrought with the power and might of a Creator.
Enchir. c. 40: Furthermore, this manner in which Christ was born of the Holy Spirit suggests to us the grace of God, by which man without any previous merits, in the very beginning of his nature, was united with the Word of God into so great unity of person, that he was also made son of God.
de Cons. Evan., ii, 5: How this was done Matthew omits to write, but Luke relates after the conception of John, “In the sixth month the Angel was sent; "and again, “The Holy Spirit shall come upon thee.” This is what Matthew relates in these words, “She was found with child of the Holy Spirit.” And it is no contradiction that Luke has described what Matthew omits; or again that Matthew relates what Luke has omitted; that namely which follows, from “Now Joseph her husband being a just man,” to that place where it is said of the Magi, that "They returned into their own country another way. "If one desired to digest into one narrative the two accounts of Christ’s birth, he would arrange thus; beginning with Matthew's words, “Now the birth of Christ was on this wise;” then taking up with Luke, from “There was in the days of Herod,”