What does Galatians 2:6 mean?

Paul has described a meeting with the influential leaders of the Christian movement in Jerusalem. At that time, Paul asked for a private meeting, with the other apostles, to describe what he had been preaching to non-Jewish people. He wanted to hear from these leaders that they were preaching the gospel of God's grace through faith in Christ, too.

His friend Titus, who was with him and not Jewish, became a test case. Would they insist that Titus be circumcised, following the law of Moses for the Jewish people, in order to be acceptable to God and included in the church? Or was Titus' faith in Jesus' death on the cross for Titus' sins enough for Him to welcome Titus into the family of God? The other apostles—the leaders of the Christian movement in Jerusalem—"added nothing" to Paul's message. In other words, they agreed with him completely. They stood with him on the all-important fact that salvation for everyone who trusts in Christ comes only by God's grace and not by following the Law.

Though the leaders and apostles themselves agreed with Paul's message of salvation by God's grace alone, there was a group in the movement who disagreed. This sect was referred to as the "Judaizers," and they claimed that Titus had to be circumcised in order to be saved (Acts 15:1). Paul called these men "false brothers." This is not a term to be taken lightly; Paul is saying that, by the definition of the gospel, they were not even Christians.

This is a detail on which Scripture is exceedingly distinct: salvation is by grace alone through faith alone, without even the slightest hint of rituals, works, or other requirements. Attempting to add any other conditions to salvation—such as circumcision or baptism—is to preach a false gospel.