2A God is a mystery, Christ is the mystery of God (Col. 2:2), and the church is the mystery of Christ (Eph. 3:4); hence, the church is actually a mystery within a mystery.
3B Our Christian living is a mystery; for example, although human love is limited, the proper love lived out by a Christian is unlimited; hence, it is a mystery—cf. v. 19a.
4C To magnify Christ is to express Christ without limitation (Phil. 1:20); it is to show the whole universe that the very Christ by whom we live is unlimited.
5D Paul's experience of Christ as his unlimited endurance was the magnification of the unlimited Christ; any attribute we have through living Christ by the bountiful supply of the Spirit of Jesus Christ will be unlimited and thus mysterious—vv. 19-21a.