This mystery is great, but I speak concerning Christ and of the assembly.
Rather, "This mystery is a great one." This profound truth, beyond man's power of discovering, but now revealed, namely, of the spiritual union of Christ and the Church, represented by the marriage union, is a great one, of deep import. See on Eph 5:30. So "mystery" is used of a divine truth not to be discovered save by revelation of God (Rom 11:25; Co1 15:51).
All Christians owe one another mutual submission in reverence for God, yielding and not lording it over each other. Where this spirit rules, husbands and wives will discharge their duties well: the church's submission to Christ teaches wives their part, and Christ's love teaches husbands theirs.
AI summary
Commenting on Ephesians 5:21-33
This is a great mystery: but I speak concerning Christ and the church. (15) That no man might dream of natural union or knitting of Christ and his Church together (such as the husbands and the wives is) he shows that it is secret, that is, spiritual and such as differs greatly from the common capacity of man.