Galatians 2:3-5

Yet not even Titus, who was with me, was compelled to be circumcised, even though he was a Greek. This matter arose because some false believers had infiltrated our ranks to spy on the freedom we have in Christ Jesus and to make us slaves. We did not give in to them for a moment, so that the truth of the gospel might be preserved for you. 

Yet not even Titus, who was with me, was compelled to be circumcised, even though he was a Greek 
Acts 16:3  Paul wanted to take him [Timothy] along on the journey, so he circumcised him because of the Jews who lived in that area, for they all knew that his father was a Greek.
1 Corinthians 9:20-21  To the Jews I became like a Jew, to win the Jews. To those under the law I became like one under the law (though I myself am not under the law), so as to win those under the law. To those not having the law (though I am not free from God's law but am under Christ's law), so as to win those not having the law. 

This matter arose because some false believers 
Acts 1:15-16  In those days Peter stood up among the believers (a group numbering about a hundred and twenty) and said, "Brothers and sisters, the Scripture had to be fulfilled in which the Holy Spirit spoke long ago through David concerning Judas, who served as a guide for those who arrested Jesus."
2 Corinthians 11:26  I have been constantly on the move. I have been in danger from rivers, in danger from bandits, in danger from my fellow Jews, in danger from Gentiles; in danger in the city, in danger in the country, in danger at sea; and in danger from false believers.

had infiltrated our ranks to spy 
Jude 4  For certain individuals whose condemnation was written about long ago have secretly slipped in among you. They are ungodly people, who pervert the grace of our God into a license for immorality and deny Jesus Christ our only Sovereign and Lord.

on the freedom we have in Christ Jesus and to make us slaves 
Galatians 5:1  It is for freedom that Christ has set us free. Stand firm, then, and do not let yourselves be burdened again by a yoke of slavery.
Galatians 5:13  You, my brothers and sisters, were called to be free. But do not use your freedom to indulge the flesh; rather, serve one another humbly in love.

We did not give in to them for a moment, so that the truth of the gospel might be preserved for you. 
Galatians 2:14-16  When I saw that they were not acting in line with the truth of the gospel, I said to Cephas [Peter] in front of them all, "You are a Jew, yet you live like a Gentile and not like a Jew. How is it, then, that you force Gentiles to follow Jewish customs? We who are Jews by birth and not sinful Gentiles know that a person is not justified by the works of the law, but by faith in Jesus Christ. So we, too, have put our faith in Christ Jesus that we may be justified by faith in Christ and not by the works of the law, because by the works of the law no one will be justified."

[brackets are mine]

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