This is from the last chapter of my second Jesus book:
The Nicene Creed, said in many churches today, was a perfected version of what the Council of Nicaea decided in 325, but it didn’t reach something like its current form until the next council at the city of Constantinople in AD381. That creed says that, “We believe in one Lord, Jesus Christ, the only Son of God, eternally begotten of the Father, God from God, Light from Light, true God from true God, begotten, not made, of one Being with the Father.”
It was now agreed that Jesus was divine in the same way that God the Father was divine. And of course it was agreed that he was human too. A Christian leader in the 300s named Gregory of Nazianzus famously argued that “what has not been assumed cannot be healed.” What he meant is that if Jesus did not really take on humanity, he can’t heal us.
So after working out the basics of the Trinity, Christians for the next hundred plus years would debate how Jesus’ humanity and divinity fit together. Now that Christianity was legal, they didn’t have to spend as much time worrying about persecution. In fact, the Roman Emperor Theodosius I made Christianity the only legal religion in 380. The contents of the Bible were also coming together. By the year 400, Christians pretty much agreed on which books belonged in the New Testament. This “peace and quiet” created a space for them to iron out some of these issues.
The end result was the common Christian belief that Jesus was both fully human and fully divine in such a way as that both were real features of who he was. The church steered a path between a number of competing ideas. For example, a bishop named Apollinarius offered the possibility that Jesus had a human body but a divine mind, as if the divine Logos had taken over a body. The result basically would suggest that Jesus didn’t have any human element in his thinking.
Still later, an archbishop named Nestorius suggested that Jesus’ almost had a split personality—a human one and a divine one. In response, a man named Eutyches argued Jesus only had one nature that was a mixture of human and divine. No doubt these were all well-intentioned attempts to figure things out, even though the politics of it all often got ugly.
The final decision, like the one about the Trinity, was very balanced. Jesus was fully human and fully divine. He was only one person (against Nestorius) but he had two natures (against Apollinarius and Eutyches). He was really tempted and experienced genuine humanity in its fullness. But he was also truly God walking on the earth.
Obviously by 451 Christians had come a long way from simple biblical statements like 2 Corinthians 13:14: “May the grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all.” The Corinthians may have had a very simple sense of such a verse. Here are three different persons: One God, one Lord, one Spirit.
When we read it today, we believe all three are mysteriously the same, one God. So Jesus the Lord is both the fully human Jesus who walked the earth and the fully divine Jesus who came from heaven. He was conceived by the Holy Spirit and born of the Virgin Mary.