Colossians 1:27 - "To them God has chosen to make known among the Gentiles the glorious riches of this mystery, which is Christ in you, the hope of glory."
The greatest mystery is God becoming man. What is in man that thou are mindful of him? So many religions struggle on the incarnation of God but with God nothing is impossible. Christ declared, "he who hath seen me, hath seen the Father."
The second greatest mystery is God coming in man. Paul declared that "Christ in you." In the same token we express once more, What is man that thou are mindful of him? There is nothing, but God-Christ comes into those who receives and willing to follow Him. "But as many as received him, to them he gave the right to become children of God..."
The third greatest mystery is that Christ is revealed beyond His children, the Israelites. Christ is now made known among the Gentiles - that is you and me. The glorious truth is declared simply in that scripture many has known as the heart of God's truth - For God so loved the world.
This is the heart of our celebration - the mystery of God - Christ in you.