The bulk of conservative evangelical Christianity believes in what is called the verbal plenary inspiration of Scripture, meaning that every single word of the Bible is “breathed out” by God (2 Timothy 3:16). If biblical critics can claim that 1 Corinthians 7:12 is not inspired, but rather Paul’s opinion, what other passages could they claim to be the opinion of the human author and not the commands of the divine Author? This strikes at the very heart of biblical authority.
Paul wrote this first letter to a group of Christians dwelling in the city of Corinth, a very corrupt city. Part of that corruption was due to the temple of Aphrodite, which was home to over 1,000 temple prostitutes who plied their trade on behalf of their deity. It was in this setting that Paul founded the church at Corinth. In fact, many of the congregation came out of the immoral Corinthian lifestyle. The church of Corinth was made up of ex-fornicators, ex-idolaters, ex-adulterers, ex-homosexuals, ex-thieves, and ex-drunkards.
With that as a backdrop, when Paul gets into chapter 7 of his letter, he is answering a question the church had regarding sexual relations between men and women. Given the social climate in Corinth, the Corinthians thought it would be a good thing for everyone to remain celibate. Paul agrees that celibacy is a good thing and even states that he wishes more people could be celibate as he was. Paul is not down on marriage. All he is doing is stating the obvious benefits that singleness affords for ministry opportunities. However, Paul mentions that singleness is a gift from God, and not all have the gift (v. 7). For those who are currently married, Paul tells them to remain so, and in v. 10 Paul says “not I, but the Lord.” This means that what Paul is telling the Corinthians is a direct command from Jesus. This command comes from Jesus’ teaching in the gospels, in particular, Matthew 5:32.
Finally, in v. 12, Paul addresses “mixed marriages”—those between a Christian and a non-Christian. Given the prevailing environment, Christians might be tempted to divorce their unbelieving spouses, thinking that somehow by doing so they are purifying themselves. Paul tells the believing spouse to remain with the unbeliever, with the comment that the command comes from him, not Jesus. But Paul is not offering his own opinion here. What he is saying is that Jesus never addressed this issue directly during His earthly ministry. If we search the Gospels, we will not see any direct teaching of Jesus that addresses the situation of a believing spouse married to an unbelieving spouse. But Jesus did give only one legitimate reason for divorce (Matthew 5:32; 19:19), and being married to an unbeliever was not it.
So the best answer is to see Paul as providing new revelation regarding an area that Jesus did not specifically address. That is why Paul says, “I, not the Lord.” In other words, I, not Jesus, am giving you this command, although it is based on the principles Jesus taught. As extensive as Jesus’ ministry was, He did not articulate everything regarding the Christian life. That is why He commissioned the apostles to carry on His ministry after His ascension, and that is why we have a God-breathed Bible, “so that the man of God may be thoroughly equipped for every good work” (2 Timothy 3:17). Paul was responsible for much new revelation, although ultimately those revelations came from the Holy Spirit. In many of his epistles, Paul reveals to us “mystery.” The word “mystery” is a technical term that signifies some previously unrevealed truth that is now revealed, such as the church being made up of both Jews and Gentiles (Romans 11:25) or the rapture (1 Corinthians 15:51-52). Paul is simply giving us additional revelation regarding marriage that Jesus didn’t elaborate on.