Question: "What does 'what God has joined together, let no one separate' mean?"
Answer: The command “what God has joined together, let no one separate” refers to marriage and divorce. It is from Jesus’ teaching on marriage and divorce found in Mark 10:1–12 and Matthew 19:1–12. On one occasion, the Pharisees asked Jesus if it is legitimate for a man to divorce his wife. Jesus in essence answers, “No”: “Haven’t you read . . . that at the beginning the Creator ‘made them male and female,’ and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate” (Matthew 19:4–6; cf. Genesis 1:27; 2:24).
Jesus’ point is that a married couple is something that “God has joined together.” Marriage is not of human origin—it originated with God and is part of the way that God designed the human race to live. In saying “let no one separate” a marriage, Jesus taught that divorce is not God’s plan. Once a couple is married, they have been joined together by God Himself, and the union is meant to be for life. This principle holds true despite the faith (or lack thereof) of the couple. When two atheists marry, they have been joined together by God, whether they recognize it or not. If God has joined them together, then no human being has the right to break that union.
Later, after Jesus says, “What God has joined together, let no one separate,” the Pharisees point out that Moses allowed divorce. Jesus agrees, but also points out that the allowance was made due to “hardness of heart” (Matthew 19:8, NASB), reiterating that divorce was never God’s original plan.
Jesus’ command against separating what God has joined implies that it is possible for a marriage union to be broken and for the one flesh to be separated by divorce. There is debate among Christians about whether divorce is ever justified. Many (perhaps most) would allow for divorce in the case of unrepentant unfaithfulness on the part of one spouse (based on Matthew 19:9) or desertion of a believing spouse by an unbelieving spouse who no longer wants to be married to a believer (see 1 Corinthians 7:15). In these cases the marriage bond has been broken by unfaithfulness or desertion—a severing of something that God has joined together—and it is a tragic occurrence.
Even if the above exceptions are allowed, our culture and, too often, even the church seem to regard divorce as something far less serious than it is. If marriage were simply a human convention similar to a business partnership or club membership, then people would be free to enter and exit at will. Divorce is not simply two people deciding to part company; it is one or perhaps both of the marriage partners deciding that they will act decisively to end something that God intended to be permanent. That is a serious thing!