It is a hypostatic union and thus is NOT a mixture of human and divine but rather both fully human and fully divine. That much I can grasp, at least theoretically; however, how in the world this idea was all pieced together and believed with certainty by the Church, albeit not without dispute, is beyond my understanding.
The Incarnation is pieced together because obviously, Adam is not on the same level as his Creator.
Adam being human does not have the power of forming an original intimate union with a transcendent, super-natural Pure Spirit. If he did have the same level of power as his Creator, he would be a duplicate God. Two equal gods is a no-no. Interestingly, in Genesis 1: 26-27, Adam is given the human power of being, not forming, in a relationship with his Creator. Adam does not become his own creator. It is the Creator and not Adam Who sets the rules for the relationship in Genesis 2: 15-17. It is obedience, not organic fruit, which is key.
The Incarnation is pieced together because God loves His human creatures.
The love of God for Adam and consequently Adam’s descendants is so powerful that a way a human could repair the damage of Original Sin is foretold in Genesis 3:15. Yet, there is a huge difficulty because Adam being human is not God and therefore he does not have the power to restore his relationship with the Person who created it. Recall that two equal gods is a no-no.
Also recall John 3-16-17. Here we see two Persons, the Father and the Son. The promised Third Person, the Holy Spirit, is described in chapter 14, Gospel of John.
God so loved us, that the Second Person of the Most Holy Trinity, the “one Lord Jesus Christ … came down from heaven and by the Holy Spirit was incarnate of the Virgin Mary, and became man.” (Niceno-Constantinopolitan Creed professed at Sunday Holy Sacrifice of the Mass) Please note that God has the power to assume Adam’s human nature. Please consider that if the Second Person changed His nature to a human nature, He would then be a human the same as Adam and therefore He could not restore the original relationship with Divinity. Recall that it is Divinity, not humanity, Who established Genesis 1: 26-27.
Go slowly with the following observations. Adam is not a Divine God so he cannot create a “new” relationship with the Divine Creator God. The Second Person of the Most Holy Trinity is God, but He is not the one who destroyed humanity’s relationship with Divinity. The human Adam broke the relationship and thus is responsible for repairing it, but he cannot do so because he is not on the same level as the Creator. God so loves humankind, that the Divine Second Person steps into the sandals of Adam and therefore the relationship can be repaired. Does wearing Adam’s sandals change God? It is God Who creates and not the creature. Therefore, Adam’s created humanity cannot change into the level of God when Adam seeks reconciliation with God.
Wearing the sandals of Adam is a simple description of the Divine Second Person Who enters our world as a human. Red flag. Very red flag! How does the Divine Second Person become human to take the place of Adam?
“How” is a huge word which is totally ignored by the title of this thread.
In order to restore a relationship with God, the Divine Second Person, known as Jesus Christ, has to be on the same level with God. His Divinity is evinced in the four Gospels. If He changes His Divinity into humanity, then He cannot “open the gates of heaven.” If changes His person into Adam, then He cannot “open the gates of heaven.” Therefore, the Person Jesus Christ has to remain Divine and not change into something else. Therefore the Divinity of Jesus Christ has to remain Divine and not change into something else.
Looking at the dramatic shift from Genesis 1: 25 to Genesis 1: 26, we see that the key difference is between the nature of animals and the nature of humans.
It is Adam’s nature, not his personal blood and guts, which will be transmitted to his descendants. It is the nature of humans which unites all humans. It is because of human nature, body and soul, that God calls each human person to share in His divine life. In reality, it is not necessary for the Savior to become Adam. It is necessary to assume human nature per se. Assuming the same human nature of Adam is why we can say that Jesus Christ is true man. Please notice that the Catholic Church does not say that Jesus Christ became the person of Adam.
Because Jesus Christ is the Second Person of the Blessed Trinity, He continues to be God. He does not have to turn into Adam to take his place. In the Gospels, Jesus does not declare he is Adam person; He says in many ways that He is God Person. It is we humans who see the nature connection between Jesus and Adam. And this is a good connection because Jesus is victorious over the human death Adam brought into the world. “Where, O death, is your sting?” 1 Corinthians: 15: 54-55.
Putting together God’s love and God’s power, the Catholic Church recognizes that the God Person, maintaining His powerful Divinity, did come into our world and did take on our (Adam’s) human nature solely out of love for all of us. Maintaining His powerful Divine Nature, Jesus took on our human nature so that His human obedience repaired the original human disobedience. Jesus, without change, is still One Divine Person with two natures, Divine and human. Thank you God.