From: Matthew 1:18-25
The Virginal Conception of Jesus, and His Birth
 Now the birth of Jesus Christ took place in this way. When His mother Mary had been betrothed to Joseph, before they came together she was found to be with child of the Holy Spirit;  and her husband Joseph, being a just man and unwilling to put her to shame, resolved to send her away quietly.  But as he considered this, behold, an angel of the Lord appeared to him in a dream, saying, "Joseph, son of David, do not fear to take Mary your wife, for that which is conceived in her is of the Holy Spirit;  she will bear a son, and you shall call His name Jesus, for He will save His people from their sins."  All this took place to fulfill what the Lord had spoken by the prophet:  "Behold, a virgin shall conceive and bear a son and His name shall be called Emmanuel" (which means God with us).  When Joseph woke from sleep, he did as the angel of the Lord commanded him; he took his wife,  but knew her not until she had borne a son; and he called his name Jesus.
18. St. Matthew relates here how Christ was conceived (cf. Luke 1:25-38): "We truly honor and venerate (Mary) as Mother of God, because she gave birth to a person who is at the same time both God and man"("St. Pius V Catechism", I, 4, 7).
According to the provisions of the Law of Moses, engagement took place about one year before marriage and enjoyed almost the same legal validity. The marriage proper consisted, among other ceremonies, in the bride being brought solemnly and joyously to her husband's house (cf. Deuteronomy 20:7).
From the moment of engagement onwards, a certificate of divorce was needed in the event of a break in the relationship between the couple.
The entire account of Jesus' birth teaches, through the fulfillment of the prophecy of Isaiah 7:14 (which is expressly quoted in verses 22-23) that: 1) Jesus has David as His ancestor since Joseph is His legal father; 2) Mary is the Virgin who gives birth according to the prophecy; 3) the Child's conception without the intervention of man was miraculous.
19. "St. Joseph was an ordinary sort of man on whom God relied to do great things. He did exactly what the Lord wanted him to do, in each and every event that went to make up his life. That is why Scripture praises Joseph as `a just man'. In Hebrew a just man means a good and faithful servant of God, someone who fulfills the divine will (cf. Genesis 7:1; 18:23-32; Ezekiel 18:5ff.; Proverbs 12:10), or who is honorable and charitable toward his neighbor (cf. Tobias 7:6; 9:6). So a just man is someone who loves God and proves his love by keeping God's commandments and directing his whole life towards the service of his brothers, his fellow men" ([St] J. Escriva, "Christ Is Passing By", 40).
Joseph considered his spouse to be holy despite the signs that she was going to have a child. He was therefore faced with a situation he could not explain. Precisely because he was trying to do God's will, he felt obliged to put her away; but to shield her from public shame he decided to send her away quietly.
Mary's silence is admirable. Her perfect surrender to God even leads her to the extreme of not defending her honor or innocence. She prefers to suffer suspicion and shame rather than reveal the work of grace in her. Faced with a fact which was inexplicable in human terms she abandons herself confidently to the love and providence of God.
God certainly submitted the holy souls of Joseph and Mary to a severe trial. We ought not to be surprised if we also undergo difficult trials in the course of our lives. We ought to trust in God during them, and remain faithful to Him, following the example they gave us.
20. God gives His light to those who act in an upright way and who trust in His power and wisdom when faced with situations which exceed human understanding. By calling him the son of David, the angel reminds Joseph that he is the providential link which joins Jesus with the family of David, according to Nathan's messianic prophecy (cf. 2 Samuel 7:12). As St. John Chrysostom says: "At the very start he straightaway reminds him of David, of whom the Christ was to spring, and he does not wish him to be worried from the moment he reminds him, through naming his most illustrious ancestor, of the promise made to all his lineage" ("Hom. on St. Matthew", 4).
"The same Jesus Christ, our only Lord, the Son of God, when He assumed human flesh for us in the womb of the Virgin, was not conceived like other men, from the seed of man, but in a manner transcending the order of nature, that is, by the power of the Holy Spirit, so that the same person, remaining God as He was from eternity, became man, which He was not before" ("St. Pius V Catechism", I, 4, 1).
21. According to the Hebrew root, the name Jesus means "savior". After our Lady, St. Joseph is the first person to be told by God that salvation has begun.
"Jesus is the proper name of the God-man and signifies `Savior'--a name given Him not accidentally, or by the judgment or will of man, but by the counsel and command of God" [...]. All other names which prophecy gave to the Son of God--Wonderful, Counsellor, Mighty God, Everlasting Father, Prince of Peace (cf. Isaiah 9:6)--are comprised in this one name Jesus; for while they partially signified the salvation which He was to bestow on us, this name included the force and meaning of all human salvation" ("St. Pius V Catechism", I, 3, 5 and 6).
23. "Emmanuel": the prophecy of Isaiah 7:14, quoted in this verse, foretold about 700 years in advance that God's salvation would be marked by the extraordinary event of virgin giving birth to a son. The Gospel here, therefore, reveals two truths.
First, that Jesus is in fact the God-with-us foretold by the prophet. This is how Christian tradition has always understood it. Indeed the Church has officially condemned an interpretation denying the messianic sense of the Isaiah text (cf. Pius VI, Brief, "Divina", 1779). Christ is truly God-with-us, therefore, not only because of His God-given mission but because He is God made man (cf. John 1:14).
This does not mean that Jesus should normally be called Emmanuel, for this name refers more directly to the mystery of His being the Incarnate Word. At the Annunciation the angel said that He should be called Jesus, that is, Savior. And that was the name St. Joseph gave Him.
The second truth revealed to us by the sacred text is that Mary, in whom the prophecy of Isaiah 7:14 is fulfilled, was a virgin before and during the birth itself. The miraculous sign given by God that salvation had arrived was precisely that a woman would be a virgin and a mother at the same time.
"Jesus Christ came forth from His mother's womb without injury to her maternal virginity. This immaculate and perpetual virginity forms, therefore, the just theme of our eulogy. Such was the work of the Holy Spirit, who at the conception and birth of the Son so favored the Virgin Mother as to impart fruitfulness to her while preserving inviolate her perpetual virginity" ("St. Pius V Catechism", I, 4, 8).
25. St John Chrysostom, addressing himself to St Joseph, comments: "Christ's conception was the work of the Holy Spirit, but do not think this divine economy has nothing to do with you. For although it is true that you had no part in the generation of Christ, and that the Virgin remained inviolate, nevertheless, what pertains to a father (not injuring the honour of virginity) that do I give you - the naming of the child. For 'you shall call his name.' Although you have not generated him, you will act as a father to him. Hence it is that, beginning with giving him his name, I associate you intimately with the one who is to be born" (Hom. on St Matthew, 4).
Following the Greek text strictly, the New Vulgate version says: "et non cognoscebat eam, donec peperit filium." The literal English translation is: "and he knew her not until she had borne a son". The word donec (until) of itself does not direct our attention to what happened afterwards; it simply points out what has happened up to that moment, that is, the virginal conception of Jesus Christ by a unique intervention of God. We find the same word in John 9:18, where it says that the Pharisees did not believe in the miraculous cure of the man blind from birth "until" (donec) they called his parents. However, neither did they believe afterwards. Consequently, the word "until" does not refer to what happens later.
The Vulgate adds after filium the words suum primogenitum, which in the Bible simply means "the first son", without implying that there are any other drildren (cf. Ex 13:2). This Latin variant gives no ground whatsoever for thinking that our Lady had other children later. See the note on Luke 2:7.
The Church has always taught that the perpetual virginity of our Lady is a truth to be held by Catholics. For example, the following are the words of the Lateran Council of A.D. 649: "If anyone does not profess according to the holy Fathers that in the proper and true sense the holy, ever-virgin, immaculate Mary, is the Mother of God, since in this last age not with human seed but of the Holy Spirit she properly and truly conceived the Divine Word, who was born of God me Father before all ages, and gave him birth without any detriment to her virginity, which remained inviolate even after his birth: let such a one be condemned" (can. 3).
St Jerome gives the following reasons why it was fitting that the Mother of God, as well as being a virgin, should also be married: first, so that Mary's child would be clearly a descendant of King David (through the genealogy of St Joseph); second, to ensure that on having a son her honour would not be questioned nor any legal penalty be imposed on her; third, so that during the flight into Egypt she would have the help and protection of St Joseph. He even points to a fourth possible reason, expressly taken from St Ignatius Martyr, and to which he seems to give less importance - that the birth of Jesus would go unnoticed by the devil, who would not know about the virginal conception of our Lord (cf. Comm. on St Matthew, 1, 1).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.