For us men and for our salvation he came down from heaven, and by the Holy Spirit was incarnate of the Virgin Mary, and became man.
The meaning of the incarnation is more complex than the story of the Virgin birth in Bethlehem.
The Word of God became flesh, i.e. Incarnate, in order to reconcile us with God, who so loved the world that he sent his Only Begotten Son, so that whoever believes in him should have eternal life. The Word became flesh to be our model of holiness. The love that Jesus demonstrates in his earthly life by offering himself for others is the essence of the Christian life.
The Son of God became human so that humanity might become divine. This is the great mystery of faith: divinity and humanity sharing a common bond.
The Fifth Council of Constantinople (553A.D.) clarified that Jesus is inseparably true God and true man. The Council Fathers declared that Jesus became man without ceasing to be God. Through the centuries the church believed the Virgin Mary was full of grace from the moment of her conception (i.e. The Immaculate Conception) and also when she conceived Jesus by the Holy Spirit (i.e. the incarnation). Thus, Mary is ever virgin and truly the Mother of Jesus and the Mother of God.
Jesus, the Word of God, became man to save us by reconciling us with the Father, so that we might know God's love, to be our model of holiness, and to make us "partakers of the divine nature".
457 The Word became flesh for us in order to save us by reconciling us with God, who “loved us and sent his Son to be the expiation for our sins”: “the Father has sent his Son as the Savior of the world", and "he was revealed to take away sins”:
Sick, our nature demanded to be healed; fallen, to be raised up; dead, to rise again. We had lost the possession of the good; it was necessary for it to be given back to us. Closed in the darkness, it was necessary to bring us the light; captives, we awaited a Savior; prisoners, help; slaves, a liberator. Are these things minor or insignificant? Did they not move God to descend to human nature and visit it, since humanity was in so miserable and unhappy a state?
458 The Word became flesh so that thus we might know God's love: “In this the love of God was made manifest among us, that God sent his only Son into the world, so that we might live through him.” “For God so loved the world that he gave his only Son, that whoever believes in him should not perish but have eternal life.”
459 The Word became flesh to be our model of holiness: “Take my yoke upon you, and learn from me.” "I am the way, and the truth, and the life; no one comes to the Father, but by me." On the mountain of the Transfiguration, the Father commands: "Listen to him!" Jesus is the model for the Beatitudes and the norm of the new law: "Love one another as I have loved you." This love implies an effective offering of oneself, after his example.
460 The Word became flesh to make us "partakers of the divine nature": "For this is why the Word became man, and the Son of God became the Son of man: so that man, by entering into communion with the Word and thus receiving divine sonship, might become a son of God." "For the Son of God became man so that we might become God." "The only-begotten Son of God, wanting to make us sharers in his divinity, assumed our nature, so that he, made man, might make men gods."
Belief in the Incarnation (the Son of God come in human flesh) is the distinctive sign of the Christian faith.
463 Belief in the true Incarnation of the Son of God is the distinctive sign of Christian faith: "By this you know the Spirit of God: every spirit which confesses that Jesus Christ has come in the flesh is of God." Such is the joyous conviction of the Church from her beginning whenever she sings "the mystery of our religion": "He was manifested in the flesh."
Jesus assumed human form in the womb of the Virgin Mary, his mother. The conception of his human body was accomplished by the action of the Holy Spirit, and not by natural generation from man, although he is truly conceived of Mary's flesh.
456 With the Nicene Creed, we answer by confessing: “For us men and for our salvation he came down from heaven; by the power of the Holy Spirit, he became incarnate of the Virgin Mary, and was made man.”
466 The Nestorian heresy regarded Christ as a human person joined to the divine person of God's Son. Opposing this heresy, St. Cyril of Alexandria and the third ecumenical council, at Ephesus in 431, confessed “"that the Word, uniting to himself in his person the flesh animated by a rational soul, became man." Christ's humanity has no other subject than the divine person of the Son of God, who assumed it and made it his own, from his conception. For this reason the Council of Ephesus proclaimed in 431 that Mary truly became the Mother of God by the human conception of the Son of God in her womb: “Mother of God, not that the nature of the Word or his divinity received the beginning of its existence from the holy Virgin, but that, since the holy body, animated by a rational soul, which the Word of God united to himself according to the hypostasis, was born from her, the Word is said to be born according to the flesh.”
484 The Annunciation to Mary inaugurates “the fullness of time”, the time of the fulfillment of God's promises and preparations. Mary was invited to conceive him in whom the “whole fullness of deity” would dwell “bodily”. The divine response to her question, “How can this be, since I know not man?” was given by the power of the Spirit: “The Holy Spirit will come upon you.”
485 The mission of the Holy Spirit is always conjoined and ordered to that of the Son. The Holy Spirit, “the Lord, the giver of Life”, is sent to sanctify the womb of the Virgin Mary and divinely fecundate it, causing her to conceive the eternal Son of the Father in a humanity drawn from her own.
486 The Father’s only Son, conceived as man in the womb of the Virgin Mary, is “Christ”, that is to say, anointed by the Holy Spirit, from the beginning of his human existence, though the manifestation of this fact takes place only progressively: to the shepherds, to the magi, to John the Baptist, to the disciples. Thus the whole life of Jesus Christ will make manifest “how God anointed Jesus of Nazareth with the Holy Spirit and with power.”
Jesus was born in Bethlehem in Judea, as written in Scripture.
423 We believe and confess that Jesus of Nazareth, born a Jew of a daughter of Israel at Bethlehem at the time of King Herod the Great and the emperor Caesar Augustus, a carpenter by trade, who died crucified in Jerusalem under the procurator Pontius Pilate during the reign of the emperor Tiberius, is the eternal Son of God made man. He ‘came from God’, ‘descended from heaven’, and ‘came in the flesh’. For ‘the Word became flesh and dwelt among us, full of grace and truth; we have beheld his glory, glory as of the only Son from the Father. . . And from his fullness have we all received, grace upon grace.’
Jesus is fully God, and fully man. As God, he has always existed with the Father and the Holy Spirit. At a specific point in history, he assumed human form and became man. He retains both of these natures fully, even now in heaven.
464 The unique and altogether singular event of the Incarnation of the Son of God does not mean that Jesus Christ is part God and part man, nor does it imply that he is the result of a confused mixture of the divine and the human. He became truly man while remaining truly God. Jesus Christ is true God and true man.
467 The Monophysites affirmed that the human nature had ceased to exist as such in Christ when the divine person of God’s Son assumed it. Faced with this heresy, the fourth ecumenical council, at Chalcedon in 451, confessed:
Following the holy Fathers, we unanimously teach and confess one and the same Son, our Lord Jesus Christ: the same perfect in divinity and perfect in humanity, the same truly God and truly man, composed of rational soul and body; consubstantial with the Father as to his divinity and consubstantial with us as to his humanity; “like us in all things but sin". He was begotten from the Father before all ages as to his divinity and in these last days, for us and for our salvation, was born as to his humanity of the virgin Mary, the Mother of God.
We confess that one and the same Christ, Lord, and only-begotten Son, is to be acknowledged in two natures without confusion, change, division or separation. The distinction between the natures was never abolished by their union, but rather the character proper to each of the two natures was preserved as they came together in one person (prosopon) and one hypostasis.
469 The Church thus confesses that Jesus is inseparably true God and true man. He is truly the Son of God who, without ceasing to be God and Lord, became a man and our brother:
“What he was, he remained and what he was not, he assumed", sings the Roman Liturgy. And the liturgy of St. John Chrysostom proclaims and sings: "O only-begotten Son and Word of God, immortal being, you who deigned for our salvation to become incarnate of the holy Mother of God and ever-virgin Mary, you who without change became man and were crucified, O Christ our God, you who by your death have crushed death, you who are one of the Holy Trinity, glorified with the Father and the Holy Spirit, save us!”
470 Because “human nature was assumed, not absorbed”, in the mysterious union of the Incarnation, the Church was led over the course of centuries to confess the full reality of Christ's human soul, with its operations of intellect and will, and of his human body. In parallel fashion, she had to recall on each occasion that Christ's human nature belongs, as his own, to the divine person of the Son of God, who assumed it. Everything that Christ is and does in this nature derives from "one of the Trinity". The Son of God therefore communicates to his humanity his own personal mode of existence in the Trinity. In his soul as in his body, Christ thus expresses humanly the divine ways of the Trinity:
The Son of God. . . worked with human hands; he thought with a human mind. He acted with a human will, and with a human heart he loved. Born of the Virgin Mary, he has truly been made one of us, like to us in all things except sin.
The Incarnation: God the Son becomes man.
461 Taking up St. John's expression, "The Word became flesh", the Church calls "Incarnation" the fact that the Son of God assumed a human nature in order to accomplish our salvation in it. In a hymn cited by St. Paul, the Church sings the mystery of the Incarnation:
Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, taking the form of a servant, being born in the likeness of men. And being found in human form he humbled himself and became obedient unto death, even death on a cross.
In the person of Jesus Christ, God reveals himself definitively and in fullness.
460 The Word became flesh to make us "partakers of the divine nature": "For this is why the Word became man, and the Son of God became the Son of man: so that man, by entering into communion with the Word and thus receiving divine sonship, might become a son of God." "For the Son of God became man so that we might become God." "The only-begotten Son of God, wanting to make us sharers in his divinity, assumed our nature, so that he, made man, might make men gods."
The Incarnation is a work of all three Persons of the Trinity.
258 The whole divine economy is the common work of the three divine persons. For as the Trinity has only one and the same nature, so too does it have only one and the same operation: “The Father, the Son and the Holy Spirit are not three principles of creation but one principle.” However, each divine person performs the common work according to his unique personal property. Thus the Church confesses, following the New Testament, “one God and Father from whom all things are, and one Lord Jesus Christ, through whom all things are, and one Holy Spirit in whom all things are”. It is above all the divine missions of the Son’s Incarnation and the gift of the Holy Spirit that show forth the properties of the divine persons.
485 The mission of the Holy Spirit is always conjoined and ordered to that of the Son. The Holy Spirit, “the Lord, the giver of Life”, is sent to sanctify the womb of the Virgin Mary and divinely fecundate it, causing her to conceive the eternal Son of the Father in a humanity drawn from her own.
Jesus is fully God, fully man.
464 The unique and altogether singular event of the Incarnation of the Son of God does not mean that Jesus Christ is part God and part man, nor does it imply that he is the result of a confused mixture of the divine and the human. He became truly man while remaining truly God. Jesus Christ is true God and true man.
The dogmas of the Blessed Virgin reveal Christ.
487 What the Catholic faith believes about Mary is based on what it believes about Christ, and what it teaches about Mary illumines in turn its faith in Christ.
The Immaculate Conception of Mary
490 To become the mother of the Savior, Mary “was enriched by God with gifts appropriate to such a role.” The angel Gabriel at the moment of the annunciation salutes her as “full of grace”. In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God’s grace.
491 Through the centuries the Church has become ever more aware that Mary, “full of grace” through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin.
The Perpetual Virginity of Mary.
495 Called in the Gospels “the mother of Jesus”, Mary is acclaimed by Elizabeth, at the prompting of the Spirit and even before the birth of her son, as “the mother of my Lord”. In fact, the One whom she conceived as man by the Holy Spirit, who truly became her Son according to the flesh, was none other than the Father's eternal Son, the second person of the Holy Trinity. Hence the Church confesses that Mary is truly “Mother of God” (Theotokos).
Mary's virginity.
496 From the first formulations of her faith, the Church has confessed that Jesus was conceived solely by the power of the Holy Spirit in the womb of the Virgin Mary, affirming also the corporeal aspect of this event: Jesus was conceived “by the Holy Spirit without human seed”. The Fathers see in the virginal conception the sign that it truly was the Son of God who came in a humanity like our own. Thus St. Ignatius of Antioch at the beginning of the second century says:
You are firmly convinced about our Lord, who is truly of the race of David according to the flesh, Son of God according to the will and power of God, truly born of a virgin,. . . he was truly nailed to a tree for us in his flesh under Pontius Pilate. . . he truly suffered, as he is also truly risen.
497 The Gospel accounts understand the virginal conception of Jesus as a divine work that surpasses all human understanding and possibility: “That which is conceived in her is of the Holy Spirit”, said the angel to Joseph about Mary his fiancee. The Church sees here the fulfillment of the divine promise given through the prophet Isaiah: “Behold, a virgin shall conceive and bear a son.”
The Virginal Marriage of Mary and Joseph (Theology of the Body 75).
Indeed, Christ's whole life, right from the beginning, was a discreet but clear distancing of himself from that which in the Old Testament had so profoundly determined the meaning of the body. Christ—as if against the expectations of the whole Old Testament tradition—was born of Mary, who, at the moment of the annunciation, clearly says of herself: “How can this be, since I know not man” (Lk 1:34), and thereby professes her virginity. Though he is born of her like every other man, as a son of his mother, even though his coming into the world is accompanied by the presence of a man who is Mary’s spouse and, in the eyes of the law and of men, her husband, nonetheless Mary’s maternity is virginal. The virginal mystery of Joseph corresponds to this virginal maternity of Mary. Following the voice from on high, Joseph does not hesitate to “take Mary...for that which is conceived in her is of the Holy Spirit” (Mt 1:20).
Even though Jesus Christ’s virginal conception and birth were hidden from men, even though in the eyes of his contemporaries of Nazareth he was regarded as “the carpenter’s son” (Mt 13:55) (ut putabatur filius Joseph: Lk 3:23), the reality and essential truth of his conception and birth was in itself far removed from what in the Old Testament tradition was exclusively in favor of marriage, and which rendered continence incomprehensible and out of favor. Therefore, how could continence for the kingdom of heaven be understood, if the expected Messiah was to be David's descendant, and as was held, was to be a son of the royal stock according to the flesh? Only Mary and Joseph, who had lived the mystery of his conception and birth, became the first witnesses of a fruitfulness different from that of the flesh, that is, of a fruitfulness of the Spirit: “That which is conceived in her is of the Holy Spirit” (Mt 1:20).
The story of Jesus’ birth is certainly in line with that “continence for the kingdom of heaven” of which Christ will speak one day to his disciples. However, this event remained hidden to the men of that time and also to the disciples. Only gradually would it be revealed to the eyes of the Church on the basis of the witness and texts of the Gospels of Matthew and Luke. The marriage of Mary and Joseph (in which the Church honors Joseph as Mary’s spouse, and Mary as his spouse), conceals within itself, at the same time, the mystery of the perfect communion of the persons, of the man and the woman in the conjugal pact, and also the mystery of that singular continence for the kingdom of heaven. This continence served, in the history of salvation, the most perfect fruitfulness of the Holy Spirit. Indeed, in a certain sense it was the absolute fullness of that spiritual fruitfulness, since precisely in the Nazareth conditions of the pact of Mary and Joseph in marriage and in continence, the gift of the Incarnation of the Eternal Word was realized.
The Son of God, consubstantial with the Father, was conceived and born as man from the Virgin Mary.