The mystery of God is God "manifest in the flesh." 1 Tim. 3:16.
But as God is manifested only in Christ, the mystery of God is God manifest in Christ in the flesh. Matt. 11:27; 1 Tim. 3:16.
And the mystery of God being God manifest in Christ in the flesh, when this mystery reaches men, it is "Christ in you, the hope of glory." Col. 1:26, 27.