This is a great mystery; either, this that was spoken before of a marriage union between Christ and the church, and its being of his flesh and of his bones, is a great mystery, and so in the latter part of the verse the apostle explains himself. Or, this that was said of the conjunction of Adam and Eve was a great mystery, (i.e. a great secret in religion), as being a type of Christ's marriage with his church; though not an instituted type appointed by God to signify this, yet a kind of natural type, as having a resemblance to it.
Related Commentaries of Ephesians 5:32
Ephesians 5:32
32 This is a great mystery: but I speak concerning Christ and the church.