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." 

When I saw that they were not acting in line with the truth of the gospel 
Galatians 2:5  We did not give in to them for a moment, so that the truth of the gospel might be preserved for you.

You are a Jew, yet you live like a Gentile and not like a Jew 
Acts 10:28  He (Peter) said to them: "You are well aware that it is against our law for a Jew to associate with or visit a Gentile. But God has shown me that I should not call anyone impure or unclean."

How is it, then, that you force Gentiles to follow Jewish customs? 
Galatians 2:12  For before certain men came from James, he used to eat with the Gentiles. But when they arrived, he began to draw back and separate himself from the Gentiles because he was afraid of those who belonged to the circumcision group. 

We who are Jews by birth 
Philippians 3:4-5  ... though I myself have reasons for such confidence. If someone else thinks they have reasons to put confidence in the flesh, I have more: circumcised on the eighth day, of the people of Israel, of the tribe of Benjamin, a Hebrew of Hebrews; in regard to the law, a Pharisee; ...

and not sinful Gentiles 
1 Samuel 15:18  "And he (God) sent you (Saul) on a mission, saying, 'Go and completely destroy those wicked people, the Amalekites; wage war against them until you have wiped them out.' "
Luke 24:7  " 'The Son of Man must be delivered over to the hands of sinners, be crucified and on the third day be raised again.' "

know that a person is not justified by the works of the law 
Romans 3:28  For we maintain that a person is justified by faith apart from the works of the law.

but by faith in Jesus Christ 
Romans 9:30-31  What then shall we say? That the Gentiles, who did not pursue righteousness, have obtained it, a righteousness that is by faith; but the people of Israel who pursued the law as the way of righteousness, have not attained their goal.

because by the works of the law no one will be justified 
Romans 4:25  He (Jesus) was delivered over to death for our sins and was raised to life for our justification.
Romans 5:1  Therefore, since we have been justified through faith, we have peace with God through our Lord Jesus Christ, ...

(parentheses are mine)

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