From: Luke 1:26-38
The Annunciation and Incarnation of the Son of God
 In the sixth month the angel Gabriel was sent from God to a city of Galilee named Nazareth,  to a virgin betrothed to a man whose name was Joseph, of the house of David; and the virgin's name was Mary.  And he came to her and said, "Hail, full of grace, the Lord is with you!"  But she was greatly troubled at the saying, and considered in her mind what sort of greeting this might be.  And the angel said to her, "Do not be afraid, Mary, for you have found favor with God.  And behold, you will conceive in your womb and bear a son, and you shall call His name Jesus.  He will be great, and will be called the Son of the Most High; and the Lord God will give to Him the throne of His father David,  and He will reign over the house of Jacob for ever; and of His Kingdom there will be no end."  And Mary said to the angel, "How can this be, since I have no husband?"  And the angel said to her, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy, the Son of God.  And behold, your kinswoman Elizabeth in her old age has also conceived a son; and this is the sixth month with her who was called barren.  For with God nothing will be impossible."  And Mary said, "Behold, I am the handmaid of the Lord; let it be done to me according to your word." And the angel departed from her.
26-38. Here we contemplate our Lady who was "enriched from the first instant of her conception with the splendor of an entirely unique holiness; [...] the virgin of Nazareth is hailed by the heralding angel, by divine command, as `full of grace' (cf. Luke 1:28), and to the heavenly messenger she replies, `Behold the handmaid of the Lord, be it done unto me according to thy word' (Luke 1:38). Thus the daughter of Adam, Mary, consenting to the word of God, became the Mother of Jesus. Committing herself wholeheartedly to God's saving will and impeded by no sin, she devoted herself totally, as a handmaid of the Lord, to the person and work of her Son, under and with Him, serving the mystery of Redemption, by the grace of Almighty God.
Rightly, therefore, the Fathers (of the Church) see Mary not merely as passively engaged by God, but as freely cooperating in the work of man's salvation through faith and obedience" (Vatican II, "Lumen Gentium", 56).
The annunciation to Mary and incarnation of the Word constitute the deepest mystery of the relationship between God and men and the most important event in the history of mankind: God becomes man, and will remain so forever, such is the extent of His goodness and mercy and love for all of us. And yet on the day when the Second Person of the Blessed Trinity assumed frail human nature in the pure womb of the Blessed Virgin, it all happened quietly, without fanfare of any kind.
St. Luke tells the story in a very simple way. We should treasure these words of the Gospel and use them often, for example, practising the Christian custom of saying the Angelus every day and reflecting on the five Joyful Mysteries of the Rosary.
27. God chose to be born of a virgin; centuries earlier He disclosed this through the prophet Isaiah (cf. Isaiah 7:14; Matthew 1:22-23). God, "before all ages made choice of, and set in her proper place, a mother for His only-begotten Son from whom He, after being made flesh, should be born in the blessed fullness of time: and He continued His persevering regard for her in preference to all other creatures, to such a degree that for her alone He had singular regard" (Pius IX, "Ineffabilis Deus," 2). This privilege granted to our Lady of being a virgin and a mother at the same time is a unique gift of God. This was the work of the Holy Spirit "who at the conception and the 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). Paul VI reminds us of this truth of faith: "We believe that the Blessed Mary, who ever enjoys the dignity of virginity, was the Mother of the incarnate Word, of our God and Savior Jesus Christ" ("Creed of the People of God", 14).
Although many suggestions have been made as to what the name Mary means, most of the best scholars seem to agree that Mary means "lady". However, no single meaning fully conveys the richness of the name.
28. "Hail, full of grace": literally the Greek text reads "Rejoice!", obviously referring to the unique joy over the news which the angel is about to communicate.
"Full of grace": by this unusual form of greeting the archangel reveals Mary's special dignity and honor. The Fathers and Doctors of the Church "taught that this singular, solemn and unheard-of-greeting showed that all the divine graces reposed in the Mother of God and that she was adorned with all the gifts of the Holy Spirit", which meant that she "was never subject to the curse", that is, was preserved from all sin. These words of the archangel in this text constitute one of the sources which reveal the dogma of Mary's Immaculate Conception (cf. Pius IX, "Ineffabilis Deus"; Paul VI, "Creed of the People of God").
"The Lord is with you!": these words are not simply a greeting ("the Lord be with you") but an affirmation ("the Lord is with you"), and they are closely connected with the Incarnation. St. Augustine comments by putting these words on the archangel's lips: "He is more with you than He is with me: He is in your heart, He takes shape within you, He fills your soul, He is in your womb" ("Sermo De Nativitate Domini", 4).
Some important Greek manuscripts and early translations add at the end of the verse: "Blessed are you among women!", meaning that God will exalt Mary over all women. She is more excellent than Sarah, Hannah, Deborah, Rachel, Judith, etc., for only she has the supreme honor of being chosen to be the Mother of God.
29-30. Our Lady is troubled by the presence of the archangel and by the confusion truly humble people experience when they receive praise.
30. The Annunciation is the moment when our Lady is given to know the vocation which God planned for her from eternity. When the archangel sets her mind at ease by saying, "Do not be afraid, Mary," he is helping her to overcome that initial fear which a person normally experiences when God gives him or her a special calling. The fact that Mary felt this fear does not imply the least trace of imperfection in her: hers is a perfectly natural reaction in the face of the supernatural. Imperfection would arise if one did not overcome this fear or rejected the advice of those in a position to help--as St. Gabriel helped Mary.
31-33. The archangel Gabriel tells the Blessed Virgin Mary that she is to be the Mother of God by reminding her of the words of Isaiah which announced that the Messiah would be born of a virgin, a prophecy which will find its fulfillment in Mary (cf. Matthew 1:22-23; Isaiah 7:14).
He reveals that the Child will be "great": His greatness comes from His being God, a greatness He does not lose when He takes on the lowliness of human nature. He also reveals that Jesus will be the king of the Davidic dynasty sent by God in keeping with His promise of salvation; that His Kingdom will last forever, for His humanity will remain forever joined to His divinity; that "He will be called Son of the Most High", that is that He really will be the Son of the Most High and will be publicly recognized as such, that is, the Child will be the Son of God.
The archangel's announcement evokes the ancient prophecies which foretold these prerogatives. Mary, who was well-versed in Sacred Scripture, clearly realized that she was to be the Mother of God.
34-38. Commenting on this passage John Paul II said: "`Virgo fidelis', the faithful Virgin. What does this faithfulness of Mary mean? What are the dimensions of this faithfulness? The first dimension is called search. Mary was faithful first of all when she began, lovingly, to seek the deep sense of God's plan in her and for the world. `Quomodo fiet?' How shall this be?, she asked the Angel of the Annunciation[...]."
"The second dimension of faithfulness is called reception, acceptance. The `quomodo fiet?' is changed, on Mary's lips, to a `fiat': Let it be done, I am ready, I accept. This is the crucial moment of faithfulness, the moment in which man perceives that he will never completely understand the `how': that there are in God's plan more areas of mystery than of clarity; that is, however he may try, he will never succeed in understanding it completely[...]."
"The third dimension of faithfulness is consistency to live in accordance with what one believes; to adapt one's own life to the object of one's adherence. To accept misunderstanding, persecutions, rather than a break between what one practises and what one believes: this is consistency[...]."
"But all faithfulness must pass the most exacting test, that of duration. Therefore, the fourth dimension of faithfulness is constancy. It is easy to be consistent for a day or two. It is difficult and important to be consistent for one's whole life. It is easy to be consistent in the hour of enthusiasm, it is difficult to be so in the hour of tribulation. And only a consistency that lasts throughout the whole life can be called faithfulness. Mary's `fiat' in the Annunciation finds its fullness in the silent `fiat' that she repeats at the foot of the Cross" ("Homily in Mexico City Cathedral", 26 January 1979).
34. Mary believed in the archangel's words absolutely; she did not doubt as Zechariah had done (cf. 1:18). Her question, "How can this be?", expresses her readiness to obey the will of God even though at first sight it implied a contradiction: on the one hand, she was convinced that God wished her to remain a virgin; on the other, here was God also announcing that she would become a mother. The archangel announces God's mysterious design, and what had seemed impossible, according to the laws of nature, is explained by a unique intervention on the part of God.
Mary's resolution to remain a virgin was certainly something very unusual, not in line with the practice of righteous people under the Old Covenant, for, as St. Augustine explains, "particularly attentive to the propagation and growth of the people of God, through whom the Prince and Savior of the world might be prophesied and be born, the saints were obliged to make use of the good of matrimony" ("De Bono Matrimonii", 9, 9). However, in the Old Testament, there were some who, in keeping with God's plan, did remain celibate--for example, Jeremiah, Elijah, Eliseus and John the Baptist. The Blessed Virgin, who received a very special inspiration of the Holy Spirit to practise virginity, is a first-fruit of the New Testament, which will establish the excellence of virginity over marriage while not taking from the holiness of the married state, which it raises to the level of a sacrament (cf. "Gaudium Et Spes", 48).
35. The "shadow" is a symbol of the presence of God. When Israel was journeying through the wilderness, the glory of God filled the Tabernacle and a cloud covered the Ark of the Covenant (Exodus 40:34-36). And when God gave Moses the tablets of the Law, a cloud covered Mount Sinai (Exodus 24:15-16); and also, at the Transfiguration of Jesus the voice of God the Father was heard coming out of a cloud(Luke 9:35).
At the moment of the Incarnation the power of God envelops our Lady--an expression of God's omnipotence. The Spirit of God--which, according to the account in Genesis (1:2), moved over the face of the waters, bringing things to life--now comes down on Mary. And the fruit of her womb will be the work of the Holy Spirit. The Virgin Mary, who herself was conceived without any stain of sin (cf. Pius IX, "Ineffabilis Deus") becomes, after the Incarnation, a new tabernacle of God. This is the mystery we recall every day when saying the Angelus.
38. Once she learns of God's plan, our Lady yields to God's will with prompt obedience, unreservedly. She realizes the disproportion between what she is going to become--the Mother of God--and what she is--a woman. However, this is what God wants to happen and for Him nothing is impossible; therefore no one should stand in His way. So Mary, combining humility and obedience, responds perfectly to God's call: "Behold, I am the handmaid of the Lord; let it be done according to your word."
"At the enchantment of this virginal phrase, the Word became flesh" ([St] J. Escriva, "Holy Rosary", first joyful mystery). From the pure body of Mary, God shaped a new body, He created a soul out of nothing, and the Son of God united Himself with this body and soul: prior to this He was only God; now He is still God but also man. Mary is now the Mother of God. This truth is a dogma of faith, first defined by the Council of Ephesus (431). At this point she also begins to be the spiritual Mother of all mankind. What Christ says when He is dying--`Behold, your son..., behold, your mother" (John 19:26-27)--simply promulgates what came about silently at Nazareth. "With her generous `fiat' (Mary) became, through the working of the Spirit, the Mother of God, but also the Mother of the living, and, by receiving into her womb the one Mediator, she became the true Ark of the Covenant and true Temple of God" (Paul VI, "Marialis Cultus", 6).
The Annunciation shows us the Blessed Virgin as perfect model of "purity" (the RSV "I have no husband" is a euphemism); of "humility" ("Behold, I am the handmaid of the Lord"); of "candor" and "simplicity" ("How can this be?"); of "obedience" and "lively faith" ("Let it be done to me according to your word"). "Following her example of obedience to God, we can learn to serve delicately without being slavish. In Mary, we don't find the slightest trace of the attitude of the foolish virgins, who obey, but thoughtlessly. Our Lady listens attentively to what God wants, ponders what she doesn't fully understand and asks about what she doesn't know. Then she gives herself completely to doing the divine will: `Behold I am the handmaid of the Lord; let it be done to me according to your word'. Isn't that marvellous? The Blessed Virgin, our teacher in all we do, shows us here that obedience to God is not servile, does not bypass our conscience. We should be inwardly moved to discover the `freedom of the children of God' (cf. Romans 8:21)" ([St] J. Escriva, "Christ Is Passing By", 173).
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.