One of the most important doctrines of the Bible is related to the two natures of Lord Jesus. For convenience this is often called the Doctrine of the Hypostatic Union.
According to this doctrine, Lord Jesus is a unique person in this Universe who is both God as well as man. He is perfect God, and perfect (sinless) man in one person. His divinity and his humanity are inseparably united with each other in such a way that He is one person, not two. At the same time, there is no admixture of the human into the divine or the divine into the human. Thus the divine nature was not changed when he became flesh or human (John 1:1,14). Instead, the divine nature was joined with the human without any confusion of nature (Colossians 2:9).
The two natures are totally separate yet act as a unit in the one person of Jesus. This is called the Hypostatic Union. While it is easy for people to think of a person as God or as man, it is difficult for the unregenerate world to think of a God-man who is fully God and fully human. Even for the regenerate people it is at times difficult to properly understand this doctrine because the coming together of the infinite divine nature with the finite human nature is not at all easy to understand or comprehend.
Implications of the Doctrine: The doctrine of God-man-saviour is very important from a number of angles. First of all, the divine involvement was essential because God wanted to bear the penalty of human sins. This would be possible only if God were the party who bore the punishment for humans sins. However, God who is perfectly righteous cannot bear such a penalty and thus a human was needed to bear sins.
God could not have created a human being afresh, because such a creation would not be from the race of Adam. At the same time, from the race of Adam He could not get a human who would be perfectly sinless as the sin nature taints the entire Adamic race. Yet, the Saviour needed to be Adamic in race so that he could bear the sins upon himself.
All of these things were satisfied when Lord Jesus (perfect God) came into this world through Mary's womb. Since He was God, human sins were borne by God Himself. Since He was born of Mary, He was from the Adamic race. Yet since He was conceived of a virgin, the human sin nature was not passed on to Him, and thus He was a sinless human being from the Adamic race. Thus Hypostatic Union and Virgin Birth together made it possible for God to freely bear the sins of mankind through a sinless son of Adam.
It was not necessary for God to go to all this length to save mankind. However, it is His infinite love towards mankind that He did so. Thus we the saved ones should always be grateful for the incarnation, the virgin-birth, and the God-man Jesus.
Distortions of the Doctrine: Natural man cannot understand the things of God because he does not have the Holy Spirit in him to make him comprehend these things. Even the committed believer often finds it difficult to grasp the full range or implications of many doctrines when the doctrine involves a fusion of the finite and the infinite. Thus many have tried to mould the doctrine of Hypostatic Union in such a manner that it fits in well with their carnal or human intellect.
Obviously, doctrines manipulated so as to fit with human taste will become heresy. Thus there are many heresies related to the Hypostatic Union. The first error is to proclaim that the two natures of Jesus have some kind of an admixture. This error is called Eutychianism. Another one says that the two natures mingled with each others to become a single nature. This heresy is called Monophysitism. The Jehovah's Witnesses focus on Jesus' humanity and ignore His divinity. The Christian Science sect emphasizes the divinity and ignores the humanity. Whenever the finite man tries to explain the infinite as per his taste and logic, errors are bound to arise. We see a lot of this in relation to the Hypostatic Union of Christ.
Summary: That God became man in Lord Jesus Christ so that God could bear the sins of mankind upon Himself is an amazing demonstration of God's love. That He had to take a human nature upon himself through virgin birth of Christ is another amazing fact of God's work for redemption. All of this should humble each one of us, motivating us to live with great commitment.