Read 2 Corinthians 6:14-18
Paul urges believers to not form partnerships with unbelievers because this might weaken their Christian commitment, integrity, or standards. It would be a mismatch. Earlier, Paul had explained that this did not mean isolating oneself from unbelievers (1 Corinthians 5). Paul even urges Christians to stay with their unbelieving spouses (1 Corinthians 7). He wanted believers to be active in their witness for Christ to unbelievers but not lock themselves into personal and business relationships that could cause them to compromise their faith. Believers should do everything in their power to avoid situations that could force them to divide their loyalties.
For those who have discovered God’s light, there can be no fellowship or compromise with darkness (1 Corinthians 10). Separation from the world involves more than keeping our distance from sinners; it means staying close to God as you can get. It involves more than avoiding worldly entertainment; it extends to how we spend our time and money. There is no way to separate ourselves totally from all sinful influences. Nevertheless, we are to resist the sin around us, without either giving up or giving in.