1} Christ had to be completely God to take the punishment of sin. Only God is holy and sinless and only He could pay the penalty.
2} Christ had to be completely human in order to be your substitute, my substitute. Because He was a man, Christ was our representative on the cross!
Pretty much, it boggles my brain how Jesus could be 100% God and 100% man. I love C. S. Lewis's analogy of this: Like an author writing himself into the script of a play, so the Word - the Author of creation - wrote Himself into the story of our world! Jesus is the author, but He also became a character for us!
Do you realize just how much Jesus gave up by becoming a man? I will never fully understand it (in this life), but Philippians 2 gives a good picture. Jesus, who was fully God, "did not regard equality with God a thing to be grasped" (2:6). In other words, Jesus didn't hold on to His position and privileges as God. Although He never stopped being God, He gave up those rights when He came to earth.
He set aside the glories of heaven; He surrendered His independent authority and became obedient to His Father; He let go of His eternal riches and lived a life of poverty. But most of all, Christ bore the wrath of God as He took the sins of the whole world on Himself. He was separated from His Father. For me. For you. This, the greatest gift, was also the greatest sacrifice ever made!
You know the rest of the story. Jesus conquered death and now God has "highly exalted Him" (2:9). Have you, in the business of this season, taken the time to exalt the Lord? Have you thanked Him for His indescribable gift? The gift that cost Him His life, gave us ours.
"For God so loved this world
Though He knew what love would cost
He made a way in a manger
To make a way to the cross"