And not only so, but we also joy in God through our Lord Jesus Christ, by whom we have now received the atonement. Romans 5:11 (KJV)
We cannot rejoice in Christ, if we do not know Him. We cannot know Him until He becomes the central desire of our hearts. He cannot become the central desire of our hearts if it was not in the Will of the Father. But we know it was in the Will of the Father, because Christ fulfilled it. Piper writes, “practically, to rejoice in God, you rejoice in what you see and know of God in the portrait of Jesus Christ. And this comes to its fullest experience when the love of God is poured out in our hearts by the Holy Spirit (Rom. 5:5).”[1] There is a subtle difference drawn here, rejoicing in God’s gifts and rejoicing in God himself. From the use of kauchaomai καυχάομαι (joy), the latter seems to be the referred meaning. It refers to the sort of pride that is attached to kinship, like family pride. (G2744)[2] The grand access that the advent grants us is this sort of joyful kinship we share with Christ. Our joy is not only in the reconciliation, but the regeneration that follows that grants us familial ties with the divine. This is the central focus of our joy.
[1] Piper, J. (2014). The Dawning of Indestructible Joy