The mystery Paul was given to proclaim was not that God would save sinners, but the basis on which sinners could stand accepted before Him.
The Fullness of God in Christ: The Incarnation and Our Redemption
Paul says the fullness of the Godhead dwells bodily in Christ. That claim is not decorative. It is what makes redemption possible at all.
God's Seal of Approval: Why Acceptance Rests on Christ Alone
The Father's approval rests upon His Son, and those united to Christ by faith share in an acceptance they could never earn for themselves.