The Incarnation is, of course, an incomprehensible miracle. So in a certain sense we cannot say “how” it happened. But we can speak about certain wrong ways of understanding it. One very common misunderstanding is that “God turned into man”. The problem with this way of speaking is that it gives the idea that God the Son lost his deity in becoming man. That is not so. He remains always “fully God” as well as “fully man”.
So the Church realized that the best way to speak of the Incarnation is to describe it as proceeding “not by conversion of the Godhead into Flesh, but by taking of the Manhood into God.” In other words, God the Son does not lose his deity, but assumes our humanity completely. God the Son does not redeem us by staying comfortably in Heaven and snapping his fingers, but by joining us completely and experiencing the whole of the human condition—its joys, labors, pleasures, humdrum moments, temptations, fears, agonies, and death from top to bottom. He lives the life of an ordinary person where most human beings have lived: not in the palaces of the elite 1%, but in the peasant daily struggle of the poor, oppressed, anonymous, and crushed in spirit. He has, he tells us, “nowhere to lay his head” (Luke 9:58).
The Church teaches us, “What has not been assumed has not been healed.” Our entire human nature—body, soul, mind, emotions, and will—is taken up into the Godhead in the person of Jesus. There is no person beyond the reach of his redemptive power, not a single sin he cannot forgive if we only ask for it, no human experience, joy, suffering, loss, or temptation with which he cannot empathize and which he has not shared personally.
There are a number of weird and fascinating consequences to this bottomless mystery. They have provided Christians with two thousand years of grist for meditation that would satisfy the most speculative science fiction fan.
One startling consequence is that the Church teaches that Jesus is not a “human person”. This often takes people aback since they assume that is a denial that Jesus is fully human.
But in fact, the logic of the Church is sound. Think of it this way: If the Trinity can be described as three “Whos” (Father, Son, and Holy Spirit) but one “What” (God), Jesus is one “Who” but two “Whats.” He is a divine Person who has assumed a human nature. He has not stopped being that divine person, nor has he turned into another—human–person. He is one divine person with two natures, divine and human. The same person who created the world–who made promises to Abraham and gave the Law to Moses–is the Son of Mary, laid in a manger, baptized in the Jordan, crucified, and raised to life again.
Another strange paradox flows from the fact that since the “human soul that the Son of God assumed is endowed with a true human knowledge”, it follows that “this knowledge could not in itself be unlimited: it was exercised in the historical conditions of his existence in space and time.” (CCC 472). So the gospels tell us that Jesus “increased in wisdom and in stature, and in favor with God and man” (Luke 2:52). He had to learn things like the rest of us. He needed to ask questions to get information he did not have. He had to stumble along making mistakes like every other kid learning a game or studying a trade or practicing his times tables. He really and truly “emptied himself” (Philippians 2:7).
Yet, mind-blowingly, the reason the omniscient God assumes a human nature with a human mind is precisely so that “’this truly human knowledge of God’s Son expressed the divine life of his person’ (Cf. St. Gregory the Great, ‘Sicut aqua’ ad Eulogium, Epist. Lib. 10, 39 PL 77, 1097A ff.; DS 475).” (CCC 473).
This is displayed in a strange paradox. On the one hand:
The human nature of God’s Son, not by itself but by its union with the Word, knew and showed forth in itself everything that pertains to God” (St. Maximus the Confessor, Qu. et dub. 66: PG 90, 840A ) Such is first of all the case with the intimate and immediate knowledge that the Son of God made man has of his Father (Cf. Mark 14:36; Matthew 11:27; John 1:18; 8:55; etc.). The Son in his human knowledge also showed the divine penetration he had into the secret thoughts of human hearts (Cf. Mark 2:8; John 2:25; 6:61; etc.). (CCC 473)
So when it is necessary for the sake of revealing God to us, Jesus’ human knowledge, in union with his omniscience as God is enabled to possess knowledge in a supernatural way. That is why we see him display prophetic foreknowledge, such as when he tells Nathanael that he saw him under the fig tree before they ever met (cf. John 1:48). He foretells his own Crucifixion and Resurrection to his disciples, predicts the betrayal by Judas and triple denial by Peter, prophecies the destruction of the temple and the city of Jerusalem, knows of the death of Lazarus before he is told of it, and can read the minds and hearts of strangers.
But in other moments, he himself freely confesses ignorance, most famously when he tells his disciples, concerning the Day of Judgment, “But of that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father only” (Matthew 24:36). How to understand this mysterious combination of omniscience and ignorance? The Catechism tells us:
By its union to the divine wisdom in the person of the Word incarnate, Christ enjoyed in his human knowledge the fullness of understanding of the eternal plans he had come to reveal (Cf. Mark 8:31; 9:31; 10:33-34; 14:18-20, 26-30). What he admitted to not knowing in this area, he elsewhere declared himself not sent to reveal (Cf. Mark 13:32, Acts 1:7). (CCC 474)
In other words, his very ignorance is revelatory. The knowledge denied him is knowledge we do not need to have. He, like we, has to live by trust in his Father, especially when the blackness closes in and he must ask the most terrible question the human soul can ask: “My God! My God! Why have you forsaken me?” (Matthew 27:46). He has plumbed the depths of abandonment experienced by the worst sinner so that the worst sinner can receive his mercy. As the Catechism puts it (CCC 470), “In his soul as in his body, Christ thus expresses humanly the divine ways of the Trinity.” (Cf. John 14:9-10)
Another strange corollary of insisting on the full humanity of Jesus is the fact that he “possesses two wills and two natural operations, divine and human” (CCC 475). That sounds bizarre, but consider the alternative. In the early Church, a group of heretics called Monothelites tried to deny Jesus a human will just as others tried to deny him a human body, a human soul, or a human mind. Again, the Church responded with the insistence that he is fully human in every way, including having a human will. That is why it really was a struggle for him in the Garden of Gethsemane to say, “Father, if you are willing, remove this chalice from me; nevertheless not my will, but yours, be done” (Luke 22:42). And that matters to us struggling sinners because “we have not a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sinning” (Hebrews 4:15). The two wills of Christ are not opposed to each other any more than his human mind is opposed to the mind of the Father. Rather, they “cooperate in such a way that the Word made flesh willed humanly in obedience to his Father all that he had decided divinely with the Father and the Holy Spirit for our salvation.” Christ’s human will “does not resist or oppose but rather submits to his divine and almighty will.”
 “Athanasian Creed”, Catholic Encyclopedia. Available on-line at http://www.newadvent.org/cathen/02033b.htm as of July 30, 2018.
 St. Gregory Nazianzus, Ep. 101, 32: SC 208, 50
 Cf. Council of Constantinople III (681): DS 556-559.
 Council of Constantinople III: DS 556.