“This mystery is profound, and I am saying that it refers to Christ and the church” (Eph 5:32). This was how Paul ended his section on marriage from Ephesians 5. And after telling wives to submit to their husbands, and husbands to lovingly lead their wives, he says that marriage is a mystery — and that the mystery of marriage revolves around the gospel.
What does he mean by this?
Tim Keller, prolific author and theologian says in his book Meaning of Marriage:
“What is the secret of marriage? Paul [says], ‘I am talking about Christ and the church’, referring to what he said earlier in verse 25, ‘Husbands love you wives just as Christ loved the church and gave himself up for her…’. In short, the secret is not simply the fact of marriage per se. It is the message that what husbands should do for their wives is what Jesus did to bring us into union with himself. And what was that?
Jesus gave himself up for us. Jesus the Son, though equal with the Father, gave up his glory and took on our human nature (Philippians 2:5ff). But further, he willingly went to the cross and paid the penalty for our sins, removing our guilt and condemnation, so that we could be in unity with him and take on his nature…Jesus’ sacrificial service to us has brought us into deep union with him and he with us. And that, Paul says, is the key not only to understanding marriage but to living it. That is why he is able tie the original statement about marriage in Genesis 2 to Jesus and the church. As one commentator put it, ‘Paul saw that when God designed the original marriage, He already had Christ and the church in mind. This is one of God’s great purposes in marriage: to picture the relationship between Christ and His redeemed people forever!’…
This is the secret — that the gospel of Jesus and marriage explain one another. That when God invented marriage, he already had the saving work of Jesus in mind.” (pp 45-47, emphasis his)