Today Piper focuses on the Deity of Christ. I think before we can understand fully the Incarnation - we have to understand the Personhood of Christ. Christ is above all and agent of Creation (Col 1), the radiance of the God the Father and the worshipped of Heaven (Heb 1). He is the I AM as He tells the Jews in the gospel of John. All these things - yet He considered Himself nothing and took on the form of a servant - the slave of a cross (Phil 2). That is remarkable. That throughout time (Alpha and Omega) Christ knew the day was coming that He would have to give up His eternal nature and take on bodily form in the womb of a teenager and live among people who would eventually nail him to a cross and mock him and spit on Him. Did He have an "advent countdown" like we do today. No matter what else - we know as Hebrews says that He endured the cross for the joy that was set before Him. As Piper starts out - that Joy was the glory of God the Father - and that is where our satisfaction must be found. I am not satisfied as I right this. There are many things I'm disappointed with in life - even just right now. Father - let me know and find my satisfaction in you - and in the Son that you sent on my behalf - that I would find my complete joy in You!
CIM talks about our citizenship. I think about Christ's citizenship - how He was a "citizen" of another "country" and He gave up those rights and freedoms (God-ness, Phil 2) to come dwell among us. And I also think of the many people I know who are out in other countries, who live and dwell in dark places, all to tell those people about this one who became a baby and died for them on a cross. These people in dark places worship other things and other gods - they don't know (or mostly even want to know) to truth of this Incarnate and Sacrificial and Glorious Jesus.
This is why we celebrate the Incarnation - because we know the other side of the Cross and what that means for the world.