And that because of false brethren unawares brought in, who came in privily to spy out our liberty which we have in Christ Jesus, that they might bring us into bondage:
Note 7 at Ga 2:4: Paul had taken Titus with him to Jerusalem as a test case of a full-fledged converted Gentile to faith in Jesus Christ. No doubt Titus manifested all the fruit of salvation without the Jewish rite of circumcision. Would the leaders of the Jerusalem church reject such a trophy of grace because of a ritual? Titus was received by the apostles, and the truth remained that people are justified by faith in Jesus Christ and not by the works of the Law (Ga 2:16).
Note 8 at Ga 2:4: There were zealous Jewish believers (Ro 10:2) who had acknowledged Christ as their Messiah but believed a person must still keep the Old Testament Law in order to be right with God. They taught faith in Christ alone was not enough. This was in direct opposition to the Gospel that Paul taught. Paul called these men false brethren. These are the ones whom he referred to in Ga 1:8-9.
These men were saying that faith alone was not enough; holiness was needed as well. Yet, in an effort to promote holiness, look at what they were guilty of. They were spying on Titus to see if he was circumcised! That's a very private matter. It can't be determined by just looking at a person. They had to have been spying on him in the latrines. They had become "peeping Toms" in the name of the Lord. They were committing a much greater sin in the name of holiness than the one they were accusing Titus of.
Likewise, legalistic Christians today often operate in anger, hatred, and cruelty with a judgmental attitude, and that is much worse than the petty traditions they accuse others of violating. Churches have been split and lives devastated over points of doctrine that were nothing more than tradition.
Note 9 at Ga 2:4: Paul made the motives of these legalistic Jews very clear. They were not motivated out of love. They were on a witch-hunt. They had already made up their minds and were looking for some way of discrediting Paul and the Gospel he preached. Ga 2:5 shows Paul's reaction.