The Bible is very explicit, specifically in the first chapter of the Gospel of John, that the Word (the second person in the Holy Trinity) became flesh. This is a central and foundational teaching within Christianity, going back to the very first Christians. However, in all the explanations of how Christ had both a divine and human nature, few have attempted to explain why the Word took on flesh.
Some will say that He did so that He might redeem humanity, but this raises the question, “Can’t God just do that without becoming human?” Certainly, if God desired, He could have simply purified humanity with a single word, but He chose not to. Why did He choose not to?
A lot of it rests upon how we were created and how natures work. All humans have a human nature, that is, we are both animal (material) and rational (immaterial). To put it in more “Sunday School” terms we each have a body and a soul. We all share this in common and it unites us all. In this unification, when one act is done all of human nature is affected by that act. If Peter steals, then human nature has stolen. This doesn’t mean that Paul is guilty or a sinner for what Peter has done, but instead that Paul’s nature is lowered to the level of Peter’s. When Adam sinned, he brought human nature down with him.
Now, God could have healed this imperfection simply by willing it to occur, but this poses a few problems:
1) The purification is involuntary – human nature would not have willingly worked towards this purification
2) The purification would leave no power for humans
3) The purification would make God completely transcendent and less than God
If we explore the first point more, what I mean by involuntary is that no one human would have uplifted human nature, so how could human nature truly improve? Such improvement would have been forced upon humanity, meaning that humans would be neither good for improving nor bad for needing improvement. If a house is on a faulty foundation and a man works to fix the foundation, we do not reward the house for having a new foundation. Likewise, if a man is a sinner, but he moves towards becoming a saint, were this done by a mere willing on God’s part, we would have no reason to reward the man (and God would have no reason to reward the man).
In participating in the human nature, Christ willingly acted as a human (for He was a human) and therefore acted in a way that brought up human nature. It opened a path that other humans could follow. Since once human acted in the whole as a perfect human, it was a human who fixed the human condition (although it was no mere human who accomplished such a task, for Christ is also divine).
When we look to the second reason, we see that as a human, in human flesh, Christ defeated the Devil. The Devil held reign over humanity because the human nature fell to him in the Garden. While he did not own us, he did have influence over us, that is, until Christ triumphed over him. This is the reason for Genesis 3:15, which states, “I will put enmity between you and the woman and between your offspring and her offspring; he shall bruise your head, and you shall bruise his heel.”
For thousands of years, the Devil bruised the heal of humanity, constantly tempting humans and exploiting their weaknesses. The human nature failed in the Garden when tempted and thus the Devil had victory over that nature. The Devil once again attempted the human nature when he tempted Christ. In Matthew 4:1 we learn that the Devil tempted Christ using the three templates of all temptation; a temptation to please man (turning inanimate objects into food), a temptation for self-glorification (by testing God), and a temptation for power (by giving Him the kingdoms). In all of these Christ resisted the temptations and in so doing uplifted human nature; as Christ had victory over the Devil while Christ was a human, human nature participated in this victory and likewise gained victory over the Devil.
The final reason Christ took on human flesh is out of His love for humanity and His immanence. While God is transcendent to humanity (that is, separated and above us), He is also immanent to us through the Son and the Spirit. By taking on human flesh, Christ became the King who experienced what His creation experienced. The Incarnation is best expressed by a parable.
A messenger came to a king crying out that the country-side was ravished with leprosy and that soon the city would succumb to the disease as well. The king, being wise and just, knew of a cure and gave that cure to the messenger. He told the messenger to distribute the cure to those who had the disease so they might be free.
In the neighboring kingdom, another messenger came to the second king and told him that the country-side was ravished with leprosy and that soon the city would succumb to the disease as well. The king, being wise and just, knew of a cure and took that cure to the masses himself. He contracted leprosy, let the disease eat away at his body, and then administered the cure and was healed. He lived as an example and suffered with his subjects, experiencing what they experience, and in so doing, made them kings in their own rights.
In the above parable, both kings are loving. Both kings are wise. Both kings are just. But the second king is more wise, more loving, and more just. The point is, God does not hold love, wisdom, and justice abstractly, as some Platonic form. Rather, these are His attributes and flow from who He is.
Since God is also perfect, He is perfectly loving, perfectly wise, and perfectly just. This means that He will act in a perfect way. While He could have simply waived His hand and freed humanity from its disease, He instead chose to suffer with humanity in order to display His love. In so doing, we have experienced His love far more than we would have if He had just waived His hand. By willingly living as a human, He became us and suffered with us and for us. By doing this, He showed us His love. To conclude, a great summary is from St. John of Damascus:
For with Him nothing is found to be done under compulsion; on the contrary, everything was done freely. Thus, it was by willing that He hungered and by willing that He thirsted, by willing that He was afraid and by willing that He died.