There can be no fellowship between believers and unbelievers when it comes to matters forbidden by the Word of God. That is Paul’s basic point in 2 Corinthians 6:14–18, where he says Christians should not be unequally yoked with unbelievers (see v. 14). Often this verse is cited to argue that Christians should not marry non-Christians, which is certainly one proper application of Paul’s teaching. After all, believers are called to marry only other believers (see 1 Cor. 7:39). However, the Apostle’s instruction applies beyond marriage. If a partnership with an unbeliever requires us to disobey the Word of God, we are unequally yoked with that non-Christian.
Paul supports his teaching on this matter in two ways. First, he asks a series of questions to which “none” is the obvious answer (vv. 14–16). One of the questions even implies that to be unequally yoked with a non-Christian is to bring Christ into a kind of accord with Satan. (Belial, a term meaning “worthless,” had come to be used by Jews for the devil himself; see v. 15.) If those who are united to Christ are in a forbidden partnership with unbelievers, then they have brought Jesus into accord with the devil.
The second way that Paul supports his teaching on the unequal yoking of believers and nonbelievers consists in a series of biblical citations. In 2 Corinthians 6:16, Paul quotes Leviticus 26:11–12, where God speaks of His dwelling among His old covenant people in the tabernacle/temple. That dwelling sanctified the old covenant sanctuary such that bringing in idols would defile it and make it unclean. Under the new covenant, believers are the temple where God makes His dwelling (1 Cor. 6:19–20; 1 Peter 2:4–6). Thus, to engage with unbelievers in such a way that we violate God’s Word defiles the new covenant temple.
In 2 Corinthians 6:17–18, Paul quotes Isaiah 52:11 and joins it to a paraphrase of 2 Samuel 7:14. The Isaiah quote calls the Jewish exiles in Babylon, especially the priests who carried the temple’s holy vessels, to leave the unclean land where they were sojourning and return to the Promised Land. The Samuel passage is God’s promise to be the Father of the Davidic king, and the promise extends to all God’s people in Christ, for by faith alone we are united to Jesus, the final Davidic king (John 1:12–13). Because God is our Father in Christ, who has also made us a royal priesthood (1 Peter 2:9), we must separate ourselves from evil just as Isaiah first called the people in exile to do.