Three clarifications before the commentary:
The New Testament does not, in any way, advocate a Christian leaving their spouse because their spouse is not a Christian. If a husband or a wife is converted to Jesus, but their spouse remains in unbelief, then it is the Christian spouse who is charged with living a life to the glory of Jesus Christ in hopes that God will use them to also save their spouse.
The New Testament seeks to show how Jesus is both fully divine and fully human. This means that any religion that affirms the historical existence of Jesus as a man, yet rejects his divine nature, is not a true religion in their view of God.
God is a trinity of persons. In other words, the “Godhead” in the Bible is defined as one God, three persons. Any religion that confuses this or rejects this does not believe in the one true God who created the heavens and the earth.
When Paul writes to the Church at Corinth and says, “Do not be unequally yoked with unbelieversâ€¦ What accord has Christ with Belial? Or what portion does a believer share with an unbeliever?” (2 Cor. 6:14-18) Paul is not defining a ‘believer’ as someone who believes in only one God. Paul is defining a believer as someone who sees Christ as God incarnate, or Christ as the God-man. Paul is talking about someone who believes the Gospel. This means that Paul would not okay a Christian (who believes Jesus is Lord) to marry a non-Christian (who rejects that Jesus is Lord).
If we do not maintain that Jesus reveals to us exactly who God is, then we cannot maintain our identity with Christ. This is the problem that arises when a Christian, who is not married, decides to marry a non-Christian. Their marriage says to the world that Jesus is not the only one who reveals the Triune God to the world. If a Christian marries a Buddhist, that marriage says that Christ and Buddha can equally reveal God to the world. If a Christian marries a Muslim, that marriage says that Christ and Muhammad both equally reveal God to the world.
But Jesus, Buddha, and Muhammad do not all reveal God to the world, only Jesus reveals God to the world. Further, according to other religions â€“ Jesus was only a man; Jesus did not die for sins in order to save those who believe in Him; Jesus was not raised from the dead.
If someone does not believe that Jesus died on a cross and was raised from the dead three days later, then the New Testament says that they are an unbeliever and someone who does not love God. The Gospel is what defines Christianity. Anything less cannot teach the truth about who God is and what he has done to save people from every tribe, nation, and tongue for Himself and for His glory.
This is why Paul says that the mystery of marriage, from the beginning, was to reveal Christ and the Church:
Ephesians 5:25-32: “Husbands, love your wives, as Christ loved the church and gave himself up for her, that he might sanctify her, having cleansed her by the washing of water with the word, so that he might present the church to himself in splendor, without spot or wrinkle or any such thing, that she might be holy and without blemish. In the same way husbands should love their wives as their own bodies. He who loves his wife loves himself. For no one ever hated his own flesh, but nourishes and cherishes it, just as Christ does the church, because we are members of his body. ‘Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh.’ This mystery is profound, and I am saying that it refers to Christ and the church.”
A Christian marrying a non-Christian cannot reveal Christ and the Church. The non-Christian does not believe in Christ or the Church and would never consider doing what Paul says. That is why Paul counsels the Corinthians never to marry an “unbeliever.”