Jesus' story did not begin with His birth in Bethlehem. John 1:14 indicates that before He "became flesh and made his dwelling among us," Jesus was "with God" and He "was God" (John 1:1). The church's teaching about the triune nature of God presupposes another equally important biblical doctrine: the dual nature of Jesus Christ.
Jesus’ human nature did not exist until He was conceived in Mary’s womb. The Gospels’ description of Christ’s birth emphasizes His humanity. Jesus was born of Mary, a virgin who was engaged to Joseph (Matt. 1:16, 18; Luke 1:27). Jesus came into the world as a child and grew to adulthood just as any ordinary child might (Luke 2:7, 40).
His divine nature always existed. The same Gospels that describe Jesus’ humanity also emphasize His divine nature by showing that Jesus did things only God can do. He performed miracles of healing, raised the dead, and forgave sins. What is the relationship between Christ’s humanity and His divinity? To explain this, the early church emphasized three important things. First, in agreement with the Gospels, they taught the genuine humanity of Jesus Christ. Second, they proclaimed His true deity. The third important truth emphasized by the church was the union of these two natures in one person.
Jesus was not an ordinary human who was elevated to divine status because of His obedience. Nor was He a God who inhabited a human body with no personality of His own. And, Jesus was not a divided being, half God and half man. In 451 ad, the Council of Chalcedon used the formula “truly God and truly man” to express the biblical truth about Christ’s two natures. They explained that these two natures were united in one person “without confusion, without change, without division, without separation.” Just as the doctrine of the Trinity teaches that God is “three in one,” one God existing in a unity of three distinct persons, Jesus is “two in one.” He possesses two natures joined together in one person.
Because He is truly human, Jesus is able to “empathize with our weaknesses” (Heb. 2:17–18). Because only He possesses the righteousness of God, Jesus can “save completely those who come to God through him” (Heb. 7:25).
To learn more, read The Person of Christ: Contours of Christian Theology by Donald Macleod (InterVarsity).