The mystery of the Incarnation, or the mystery of God’s becoming man, is not something that can easily be understood by finite human minds. However, just as we can’t fully comprehend God himself but can at least understand him at a certain elementary level, so can we also have a basic understanding of how it is not logically impossible for God to become man in the divine person of Jesus Christ.
As we have already seen, Christians believe that God is a Trinity of Father, Son, and Holy Spirit, who each fully possess the divine nature. Upon his Incarnation, God the Son became man within the body of his mother, Mary. He did all this while remaining one divine person and retaining his fully divine nature, even as he took on a fully human nature through the Incarnation. This mystery is called the hypostatic union, and it means that Jesus was not half God and half man (like a Greek demigod) but was 100 percent God and 100 percent man. How is this possible? According to the Catechism:
Christ’s human nature belongs, as his own, to the divine person of the Son of God, who assumed it. Everything that Christ is and does in this nature derives from “one of the Trinity.” The Son of God therefore communicates to his humanity his own personal mode of existence in the Trinity. In his soul as in his body, Christ thus expresses humanly the divine ways of the Trinity (CCC 470).
Saying that “Jesus is God” means that Jesus is a divine person. Whatever is true of Jesus is also true of God, even if it may sound strange at first. For example, since Jesus died on the cross, it is also true that God died on the cross, because Jesus is God. Of course, God did not go out of existence,but that is not what it means to die.
Death occurs when a being’s parts are separated into their basic elements. In this case, Jesus’ soul was separated from his body, but God still raised Jesus from the dead (Rom. 10:9). Since Jesus is God, then it is true that Jesus raised himself from the dead, just as he said he would in John 2:19-21.
The Catechism goes on to say that even though Christ had a divine nature, his knowledge was true human knowledge; as such, it was limited, and Jesus had to go through a process of learning to acquire it (CCC 472). This is echoed in Luke 2:52, where it says that, “Jesus increased in wisdom and in stature, and in favor with God and man.” Christ had human knowledge and a human will that cooperated with the Father, but he also possessed a divine will and divine knowledge as a part of his divine nature.
The universe continued to function while Christ was an infant because even though the Son’s divine nature was united to his human nature, it was not limited by it. In his divine nature, God the Son continued to operate in perfect communion with the other two divine persons of the Holy Trinity. So it is accurate to say that God, in the person of the Son, became a little baby by taking on a human nature. This doesn’t mean, however, that a little baby possessing only a human nature was put in charge of ordering the universe. Instead, God the Son, who was fully divine throughout his entire human life on earth, sustained and still sustains, all of existence.