This is a great question! In fact it’s the second major question about Jesus that preoccupied the best minds in the church in the early centuries of Christendom.
The first major question about Jesus was how he could be God and address God as Father at the same time. What did these biblical statements infer about the nature of God? The Church eventually settled on a Trinitarian formula – that God was one in being, three in person, Father, Son and Holy Spirit.
It is sometimes asserted that because this first question wasn’t fully settled until the 4th century that this means the Church essentially picked this view of a fully deified Son out of thin air, and foisted this late developing view onto history. “Da Vinci Code” anyone?
But history shows that Christ’s divinity was never in doubt by the Church because, despite its disparate views on Christ, all sides were deriving their view fundamentally from the 4 gospels and the letters of Paul – all first century documents. The 2nd century Gnostics were not really serious players in the debate, but even they never doubted that Jesus was divine.
The questions were about exactly HOW the Father shared his divinity with the Son – was the Son a mode of God, or the highest created thing in which God’s nature was fully reflected, or fully God? This unanimity on the main point is due to the fact that the debates were anchored, not in the whims of church leaders, but the New Testament documents themselves.
Like scientists who disagreed about which theory captures the data best but agree on the data itself, the early church mostly agreed that those earliest documents were their trusted data field. So in the end, each view they rejected lost out because there was scriptural data it could not fully incorporate. The view that “won” – the Trinity – was the view that captured ALL the data.
So once the Church articulated the Trinitarian nature of God, they then turned to your question: Since the Bible is clear that Jesus is fully God, and equally clear that he lived as a normal human man, how do you fuse those two natures? After they answered how God could be three and one, they asked, how can a person be both truly a human being and truly God?
For starters, it was taken for granted that the reason for the Incarnation was that God had to assume human nature in order to save it. He could not put right what was wrong in us, if he did not take on a complete human nature to redeem it.
So the first way that Christians put these two natures together was to say Jesus was like us, with a body, a mind and a spirit, but that the Second Person of the Trinity displaced the human spirit of Jesus the man. But the Church soon realized that if Jesus did not have a human spirit, then he wasn’t TRULY a human. That meant God could not redeem and save the human spirit. So this view was eventually rejected.
Another view, Nestorianism, came to the fore. This agreed there had to be a full human and divine nature in Christ, but saw them as two persons fused in Jesus. But this too was condemned because it imagined two actual persons within Jesus nature. Scripture did not show a Christ who had two personalities, talking to each other, one possessing the other etc. Jesus is clearly one person.
So after the Council of Chalcedon in 451, the Church established two bumpers to define a biblical position. They developed a motto, “Never confusing the natures, never dividing the person.” They formed a Creed as follows:
We confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man … one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, without confusion, without change, without division, without separation; the difference of the natures being by no means taken away because of the union, but rather the property of each nature being preserved, and concurring in one Person …not divided or separated into two persons, but one and the same Son, and only begotten God, Word, Lord Jesus Christ.
So, condemning the view of God displacing a human nature, the Church also condemned the 2-persons-in-one view. That left their Creed, two complete natures, one person.
Now, they left it open as to the HOW, yet the basic idea was clear. If you cut through all the debate, the overly-careful redundancy of the Creed, the fancy words, like “consubstantial” and “subsistence” etc., the Church finally landed in a very simple place:
They acknowledged the biblical minimums without committing to confusing speculation. Those minimums boil down to this: A man came who was really and wholly a human man and yet he claimed to be God, having all the prerogatives and qualities of God. Yet this man never showed signs of being possessed, internally conflicted or schizophrenic – a cohesive, complete, undivided, singular person.
So he was two and yet he was one.
Now, as to the “how” question, more modern debate has centered on Philippians 2 for help on that. Paul there states:
Phil 2:7: Instead He emptied Himself by assuming the form of a slave, taking on the likeness of men.
The key Greek word is “kenosis” – to empty. This suggests that in order for a full divine nature to fuse with a human nature in one person, the Son was emptied of some divine privileges in the Incarnation. And this perhaps comes as close as we can get to answering the “HOW” question regarding the seeming impossibility of an infinite God merging with finite humanity.
However, even here we must be very careful how much we lean on that one word “emptying” – since if Jesus was emptied of any core aspect of deity or any essential divine attributes, he ceased to be God. God cannot be less than God and still be God.
So we should say that by becoming human, the Son emptied himself of some divine privileges, yet he did this willfully and only temporarily WITHOUT losing divine attributes. To go back to what was established at Chalcedon, we can say that what is true of one nature was not always true of the other, yet whatever was true of one nature had to be true for Jesus, the one Person.
And that best makes sense of everything Jesus said and did. For example, when Jesus claimed the Father was greater than he (John 14:28), that was certainly true of his human nature, but it was not true of his divine nature. Yet the one Person Jesus experienced the Father’s greatness and lived in submission to it. This is part of the Kenosis.
Another example: when Jesus claimed that he didn’t know the hour of his return (Matt 24:36), that was true in his human nature. But his divine nature was omniscient (John 21:17). So he was emptied and therefore didn’t consciously have access to all knowledge during his life on earth. Also when Jesus died, he could die because he had a human nature, but his divine nature could not die. So the “emptying” of Phil 2 gives us the reason that some aspects of His divine nature were ‘hidden’ to him or left “unconscious”.
And yet – and here is the beautiful mystery – because Jesus is one Person his divine nature which cannot die, tasted death, so that he experienced through his human nature everything we as humans do. Why? For this purpose:
Because God’s children are human beings—made of flesh and blood—the Son also became flesh and blood. For only as a human being could he die, and only by dying could he break the power of the devil, who had the power of death. 15 Only in this way could he set free all who have lived their lives as slaves to the fear of dying.Heb 2:14-15