The Incarnation is the mystery of the Word made Flesh. ln this technical sense the word incarnation was adopted from the Latin incarnatio. The Church calls "Incarnation" the fact that the Son of God assumed a human nature in order to accomplish our salvation in it. Consequently, when Christ came into the world, he said, "Sacrifices and offerings you have not desired, but a body have you prepared for me; in burnt offerings and sin offerings you have taken no pleasure. Then I said, Lo, I have come to do your will, O God." ( Hebrews 10:5 and Psalm 40:7). Belief in the true Incarnation of the Son of God is the distinctive sign of Christian faith. And according to John 3:16: "God so loved the world as to give His only-begotten Son, that whosoever believeth in Him may not perish, but may have life everlasting." Therefore it was necessary for man's salvation that God should become incarnate. The Latin Fathers, from the fourth century, make common use of the word (Saints Jerome, Ambrose, Hippolytus, Hilary and others).
I. The Fact of the Incarnation
The Incarnation implies three facts: (1) The Divine Person of Jesus Christ; (2) The Human Nature of Jesus Christ; (3) The Hypostatic Union of the Human with the Divine Nature in the Divine Person of Jesus Christ.
(1) The Divine Person of Jesus Christ
He was a real person of history; the Messiahship of Jesus; the historical worth and authenticity of the Gospels and Acts; the Divine ambassadorship of Jesus Christ established thereby; the establishment of an infallible and never failing teaching body to have and to keep the deposit of revealed truth entrusted to it by the Divine ambassador, Jesus Christ; the handing down of all this deposit by tradition and of part thereof by Holy Writ; the canon and inspiration of the Sacred Scriptures
A. Old Testament Proofs
Assuming then, that Jesus is the Christ, the Messiah promised in the Old Testament, from the terms of the promise it is certain that the One promised is God, is a Divine Person in the strictest sense of the word, the texts from the Old Testament have weight by themselves; taken together with their fulfilment in the New Testament, to make up a cumulative argument in favour of the Divinity of Jesus Christ that is overwhelming in its force. The Old Testament proofs we draw from the Psalms, the Sapiential Books and the Prophets. They are far too numerous to mention them all here. (For Example: Psalm 2:7. "The Lord hath said to me: Thou art my son, this day have I begotten thee. and "I will be to him a father, and he shall be to me a son" from II Kings 7: 14. It is to be noted that in the pre-sapiential books of the Old Testament, the uncreated Logos, or hrema, is the active and creative principle of Yahweh (see Ps. 32: 4, 6; 115: 89; 102: 20; Isaiah 40: 8; 60: 11). Later the logos became sophia, the uncreated Word became uncreated Wisdom. To Wisdom were attributed all the works of creation and Divine Providence (see Job 28: 12: Prov. 8 and 9 Ecclus. 1:1; 24: 5-12; Wis.6: 21; 9: 9). In Wis.9: 1-2 we have a remarkable instance of the attribution of God's activity to both the Logos and Wisdom. In 9: 6, Isaiah calls the Messiah God: "A child is born to us . . . his name shall be called Wonderful Counsellor, God the Strong One, the Father of the world to come, the Prince of Peace." Catholics explain that the very same child is called God the Strong One (Isaiah 9: 6) and Emmanuel (Isaiah 7: 14); the conception of the child is prophesied in the latter verse, the birth of the very same child is prophesied in the former verse.)
B. New Testament Proofs
The argument from the New Testament has a cumulative weight that is overwhelming in its effectiveness, once the inspiration of the New Testament and the Divine ambassadorship of Jesus are proved. The Divinity of the Messiah as fulfilled in Matt.1: 23; 2: 6: Mark 1: 2: Mark 3: 12: Luke 7: 27, and many others. Also, Jesus Himself clearly assumed the title. He constantly spoke of God as "My Father" (Matt.7: 21; 10: 32; 11: 27; 15: 13; 16: 17, etc.). Jesus also said "he that sees me sees the Father" (John 14: 9).
C. Witness of Tradition
The two main sources wherefrom we draw our information as to tradition, or the unwritten Word of God, are the Fathers of the Church, certain pagan historians, and the general councils. The Fathers are practically unanimous in explicitly teaching the Divinity of Jesus Christ, among them are St. Clement of Rome (A.D. 93-95), St. Ignatius of Antioch (A.D. 110-117), Saint Justin Martyr (A.D. 150), St. Clement of Alexandria (A.D. 150) and others too numerous to mention here. To the witness of these Fathers of the Apostolic and apologetic age, there are witnesses from the pagan writers of tht era such as: Pliny (A.D. 107), Emperor Hadrian (A.D. 117), and Aristides (A.D. 138-161). The first general council of the Church was called to define the Divinity of Jesus Christ and to condemn Arius and his errors. The Council of Nicaea in 325 A.D. defined the Divinity of Christ in the clearest terms: "We believe . . . in one Lord Jesus Christ, the Son of God, the Only-begotten, God of God, Light of Light, True God of True God, begotten not made, the same in nature with the Father by Whom all things were made".
(2) The Human Nature of Jesus Christ
The title that is characteristic of Jesus in the New Testament is Son of Man; it occurs some eighty times in the Gospels; it was His Own accustomed title for Himself. The phrase is Aramaic, and would seem to be an idiomatic way of saying "man". The life and death and resurrection of Christ would all be a lie were He not a man, and our Faith would be vain. (I Cor.15: 14). "For there is one God, and one mediator of God and men, the man Christ Jesus" (I Tim.2: 5). Why, Christ even enumerates the parts of His Body. "See my hands and feet, that it is I myself; touch and see: for a spirit has not flesh and bones, as you see me to have" (Luke 24: 39). St. Augustine says, in this matter: "If the Body of Christ was a fancy, then Christ erred; and if Christ erred, then He is not the Truth. But Christ is the Truth; hence His Body was not a fancy'. In regard to the human soul of Christ, the Scripture is equally clear. Only a human soul could have been sad and troubled. Christ says: "My soul is sorrowful even unto death" (Matt. 26: 38). "Now is my soul troubled" (John 12: 27). His obedience to the heavenly Father and to Mary and Joseph supposes a human soul (John 4: 34; 5: 30; 6: 38; Luke 22: 42). Finally Jesus was really born of Mary (Matt. 1: 16), made of a woman (Gal. 4: 4), after the angel had promised that He should be conceived of Mary (Luke 1: 31); this woman is called the mother of Jesus (Matt. 1: 18; 2: 11; Luke 1: 43; John 2: 3); Christ is said to be really the seed of Abraham (Gal. 3: 16), the son of David (Matt. 1: 1).
(3) The Hypostatic Union
We all know about this one. But to be clear about it we speak here of no moral union, no union in a figurative sense of the word; but a union that is physical, a union of two substances or natures so as to make One Person, a union which means that God is Man and Man is God in the Person of Jesus Christ.
A. The Witness of the Scriptures
John says: "The Word was made flesh" (1: 14), that is, He Who was God in the Beginning (1: 2), and by Whom all things were created (1: 3), became Man. According to the testimony of St. Paul, the very same Person, Jesus Christ, "being in the form of God emptied himself, taking the form of a servant" (Phil. 2: 6, 7). It is always one and the same Person, Jesus Christ, Who is said to be God and Man, or is given predicates that denote Divine and human nature. The author of life (God) is said to have been killed by the Jews (Acts 3: 15); but He could not have been killed were He not Man.
B. Witness of Tradition
The early forms of the creed all make profession of faith, not in one Jesus Who is the Son of God and in another Jesus Who is Man and was crucified, but "in one Lord Jesus Christ, the Only-begotten Son of God, Who became Man for us and was crucified". The forms vary, but the substance of each creed invariably attributes to one and the same Jesus Christ the essence of the Godhead and of man.
II. The Nature of the Incarnation
Now we deal with the question of the nature of this fact, the manner of this tremendous miracle, the way of uniting the Divine with the human nature in one and the same Person. First I will point out several heresies that pertain to the nature of the Incarnation and how the Church dealt with them.
A. Gnostic Docetism
This first heresy denied not so much Christ's divinity as his true humanity. From apostolic times the Christian faith has insisted on the true incarnation of God's Son "come in the flesh". In the third century, the Church in a council at Antioch had to affirm against Paul of Samosata that Jesus Christ is Son of God by nature and not by adoption.
The first ecumenical council of Nicaea in 325 confessed in its Creed that the Son of God is "begotten, not made, of the same substance (homoousios) as the Father", and condemned Arius, who had affirmed that the Son of God "came to be from things that were not" and that he was "from another substance" than that of the Father.
Nestorius, Patriarch of Constantinople in 428 A.D., called the union of the two natures a mysterious and an inseparable joining (symapheian), but would admit no unity (enosin) in the strict sense of the word to be the result of this joining. As is usual in these Oriental heresies, the metaphysical refinement is faulty, and leads to a practical denial of the mystery. The oneness of the Person was percieved as only moral, and not at all physical - which denies the Hypostatic Union. St. Athanasius in about 350 A.D. said: "They err who say that it is one person who is the Son that suffered, and another person who did not suffer...". The Council of Ephesus in 431 A.D. condemned the heresy of Nestorius.
Nestorius had postulated two persons in Jesus Christ. A new heresy soon began. It postulated only one Person in Jesus, the Divine Person. The new heresy defended only one nature, as well as one Person in Jesus. The leader of this heresy was Eutyches. His followers were called Monophysites. They varied in their ways of explanation. Some thought the two natures were intermingled into one. Others are said to have worked out some sort of a conversion of the human into the Divine. All were condemned by the Council of Chalcedon in 451 A.D. This Fourth General Council of the Church defined that Jesus Christ remained, after the Incarnation, "perfect in Divinity and perfect in humanity . . . consubstantial with the Father according to His Divinity, consubstantial with us according to His humanity . . . one and the same Christ, the Son, the Lord, the Only begotten, to be acknowledged in two natures not intermingled, not changed, not divisible, not separable"
Three patriarchs of the Oriental Church gave rise to a new heresy. These three patriarchs were Sergius, the Patriarch of Constantinople, Cyrus, the Patriarch of Alexandria, and Athanasius, the Patriarch of Antioch. They denied that Christ had two wills.Their error was condemned by the Sixth General Council (also known as the Third Council of Constantinople) in 680 A.D. This refutation defined that in Christ there were two natural wills and two natural activities, the Divine and the human, and that the human will was not at all contrary to the Divine, but rather perfectly subject thereto The error of Monothelism is clear from the Scripture as well as from tradition. Christ did acts of adoration (John 4: 22), humility (Matt.11: 29), reverence (Heb. 5: 7). These acts are clearly those of a human will. The Monothelites denied that there was a human will in Christ. Recall that Jesus prayed: "Father, if Thou wilt, remove this chalice from me: but yet not my will, but thine be done," (Luke 22: 42). Here there is question of two wills, the Father's and Christ's. The will of Christ was subject to the will of the Father.
(2) The Truth
It is to be remembered that, when the Word took Flesh, there was no change in the Word; all the change was in the Flesh. At the moment of conception, in the womb of the Blessed Mother, through the forcefulness of God's activity, not only was the human soul of Christ created but the Word assumed the man that was conceived. For example, when God created the world, the world was changed, that is, it passed from the state of nonentity to the state of existence; and there was no change in the Logos or Creative Word of God the Father. Nor was there change in that Logos when it began to terminate the human nature. A new relation ensued; but this new relation implied in the Logos no new reality or change; all new reality, all real change, was in the human nature.
III. Effects of the Incarnation
(1) On Christ Himself
A. On the Body of Christ
Catholics hold that, before the Resurrection, the Body of Christ was subject to all the bodily weaknesses to which human nature unassumed is universally subject; such are hunger, thirst, pain, death. Christ hungered (Matt. 4: 2), thirsted (John 29: 28), was fatigued (John 4: 6), suffered pain and death. "We have not a high priest, who cannot have compassion on our infirmities: but one tempted in all things like as we are, without sin" (Heb. 4: 15). "For in that, wherein he himself hath suffered and been tempted, he is able to comfort them also that are tempted" (Heb. 2: 18). All these bodily weaknesses were not miraculously brought about by Jesus; they were the natural results of the human nature He assumed. The reasonableness of these bodily imperfections in Christ is clear from the fact that He assumed human nature so as to satisfy for that nature's sin. Now, to satisfy for the sin of another is to accept the penalty of that sin. Hence it was fitting that Christ should take upon himself all those penalties of the sin of Adam that are common to man and becoming. or at least not unbecoming to the Hypostatic Union. (See The Summa Theologica III:14 for other reasons.) Since the Word became flesh in assuming a true humanity, Christ's body was finite. Therefore the human face of Jesus can be portrayed; at the seventh ecumenical council in Nicaea in 787 A.D. the Church recognized its representation in holy images to be legitimate. And St. Thomas Aquinas said in his Summa Theologica 3:1: "It would seem most fitting that by visible things the invisible things of God should be made known; for to this end was the whole world made, as is clear from the word of the Apostle in Romans 1:20): 'For the invisible things of God . . . are clearly seen, being understood by the things that are made'."
B. On the Human Soul and Will of Christ
This human soul that the Son of God assumed is endowed with a true human knowledge. As such, this knowledge could not in itself be unlimited: it was exercised in the historical conditions of his existence in space and time. This is why the Son of God could, when he became man, "increase in wisdom and in stature, and in favour with God and man",  and would even have to inquire for himself about what one in the human condition can learn only from experience. This corresponded to the reality of his voluntary emptying of himself, taking "the form of a slave". The effect of the Incarnation on the human will of Christ was to leave it free in all things save only sin. It was absolutely impossible that any stain of sin should soil the soul of Christ. Neither sinful act of the will nor sinful habit of the soul were in keeping with the Hypostatic Union. The fact that Christ never sinned is an article of faith where the sinlessness of Christ is implicit in the definition that he did not offer Himself for Himself, but for us. This fact of Christ's sinlessness is also evident from the Scripture. "There is no sin in Him" (I John 3: 5).
C. On the God-Man
One of the effects of the union of the Divine nature and human nature in One Person is a mutual interchange of attributes, Divine and human, between God and man, the Communicatio Idiomatum. We may say God is man, was born, died, was buried. This refers to the Person Whose nature is human, as well as Divine; to the Person Who is man, as well as God. We do not mean to say that God, as God, was born; but God, Who is man, was born. You may say that Jesus is God; Jesus is man; the God-Man was sad; the Man-God was killed, but some ways of speaking should not be used, not that they may not be rightly explained, but that they may easily be misunderstood in an heretical sense
(2) The Adoration of the Humanity of Christ
The human nature of Christ, united hypostatically with the Divine nature, is adored with the same worship as the Divine nature. We adore the Word when we adore Christ the Man; but the Word is God. The human nature of Christ is not at all the reason of our adoration of Him; that reason is only his Divine nature. The entire term of our adoration is the Incarnate Word; the motive of the adoration is the Divinity of the Incarnate Word.
(3) Other Effects of the Incarnation
The effects of the incarnation on the Blessed Mother and us include Grace, Justification, Salvation, Satisfaction, and the Immaculate Conception. The Word became flesh so that thus we might know God's love. The Word became flesh to be our model of holiness, so that we might attain to holiness. The Word became flesh for us in order to save us by reconciling us with God.Properly speaking, the office of a mediator is to join together and unite those between whom he mediates: for extremes are united in the mean [medio]. Now to unite men to God perfectively belongs to Christ, through Whom men are reconciled to God, according to 2 Cor. 5:19: "God was in Christ reconciling the world to Himself." And, consequently, Christ alone is the perfect Mediator of God and men, inasmuch as, by His death, He reconciled the human race to God. Hence the Apostle, after saying, "Mediator of God and man, the man Christ Jesus," added: "Who gave Himself a redemption for all."
The Introduction by C.S. Lewis - Pages 3-4
Paragraphs 46 & 47 - The Refutation of the Gentiles (continued)
Page 19 - "...the singular love of God towards Man..."
Page 27 - "...more beloved in one century..."
Page 33 - "...all greatness together is but the smallest atom..."
Page 44 - "...the prayer of the Mother of God has the force of a command..."
Page 86 - "...the greatest privilege of God..."
Page 121 - "...born in a stable, open to the public road..."
Page 128 - "...God and clay, majesty and weakness..."
Page 135 - "...to raise us, he lowered himself..."
ST.ATHANASIUS, De Incarnatione Verbi Dei (The Incarnation of the Word of God)
ST. ALPHONSUS de LIGUORI, The Incarnation, Birth and Infancy of Jesus Christ
ST. THOMAS, Summa Theologica - III
THE CATHOLIC CHURCH, The Catechism - Section Two Article Three - Paragraphs 456-483
ENCYCLOPEDIA PRESS, INC., The Catholic Encyclopedia - The Incarnation
GOD, The Holy Bible - Various