I command the married–not I, but the Lord–a wife is not to leave her husband. But if she does leave, she must remain unmarried or be reconciled to her husband–and a husband is not to leave his wife. (1 Corinthians 7:10-11; HCSB)
What part of this command is difficult? When a person gets married, the couple is to remain married. If divorce does occur, they must remain unmarried or be reconciled to each other. We are not allowed to divorce and remarry for any reason. God has specifically condemned divorce and remarriage, except for a situation of a spouse cheating (Matthew 19:9). The phrase “not I, but the Lord” means that Paul is reiterating what Jesus taught on marriage and divorce while he was on the earth (Matthew 5:32; 19:1-9).
I don’t think Paul or Jesus could have been any clearer.