The Bible does not explicitly address the question of whether Jesus Christ has two natures or only one. As it will be explained below, however, understanding that Christ has two natures is the most biblically and theologically consistent position. The issue came to a head in church history as theologians in the church tried to grapple with and codify the information that the New Testament provides about Jesus.
According to the New Testament, Jesus really is a man, born into the human race, yet He is also fully God. John 1:1 states that the Word is God and then in verse 14 we see that the Word John is speaking of is Jesus who took on human flesh and “tabernacled” among us. Matthew and Luke both tell of Jesus’ birth of the Virgin Mary and give His human lineage. It is difficult to understand and explain, but that is what the New Testament teaches. Jesus is God who entered the human race as a man.
Some groups early on tried to explain the nature of Christ by saying that the divine “Christ spirit” came upon the man Jesus. Early Gnostics said that the Christ spirit came upon Jesus at His baptism and left Him at the crucifixion. In this scenario, it might seem as though Jesus had two natures; however, on closer examination, this is not the case. The man that people identified as Jesus would actually be two persons sharing a body, and each person would only have one nature. He would be Jesus the human and Christ the divine. In this scenario, God only appears to enter the human race, but He does not actually do it.
Another way of trying to explain the data in the New Testament is to say that Jesus Christ was only one person AND that He only had one nature. The difficulty with this explanation is that His nature would be something of an amalgamation of divine and human. He would not be fully human because the divine nature has mixed with the human nature, making Him something more than human. He would not be fully God because the human nature has mixed with the divine nature, making Him something less than divine. We see parallels to this idea in Greek and Roman mythology where a god has a child with a human woman. The offspring is more than human and less than a god—a super human or a demi-god. Hercules was one such person, the son of Zeus and the woman Alcmene.
An illustration may be helpful. Like most illustrations, it is far from perfect and cannot be pressed on every point. Suppose a king wants to identify with the poorest in his country. One way he could do it would be to disguise himself as a beggar and move among them. However, in this situation he is only pretending to be a beggar; he can go back to the castle at night, and he still has all the resources of a king. On the other hand, the king could renounce his throne and give away everything and become a beggar. But in this case, he would cease to be a king. A third option is that he could, for a time, give up the use of all his resources for a set period of time—let’s say 3 years—knowing that at the end of that time he would once again resume the throne. In this last situation, he is both truly a beggar and truly a king. Jesus became man, but He remained God.
The only way to adequately explain the biblical data is to say that Jesus is one Person with two natures—a human nature and a divine nature. He is both God and Man. His two natures are inseparably united (not mixed) in what theologians term the “hypostatic union.” The New Testament affirms that Jesus Christ, who walked the earth, died on a cross, and rose again, was fully a member of the human race with a fully functioning human nature (without sin). At the same time, Jesus was fully God. He willingly humbled Himself and gave up His glory and the right to use His divine attributes apart from the direction of God the Father, but He never ceased to be God. Jesus Christ is fully man and fully God—He has the nature of both. He is a man, but He is more; He is also God. He is God, but He has forever joined Himself to a human nature. A shortened way to express this is to refer to Jesus as the God-Man. He is the Man who is also God, and He is God who became a Man.