Paul writes Galatians
Hearing that teachers were persuading his Galatian converts that Gentiles must be circumcised and keep the Law to be saved, Paul wrote in alarm: we are justified by faith in Christ, not by the works of the law.
yet knowing that a man is not justified by the works of the law but through faith in Jesus Christ, even we believed in Christ Jesus, that we might be justified by faith in Christ and not by the works of the law, because no flesh will be justified by the works of the law.
Galatians 2:16 (World English Bible)
Paul told how he had even opposed Peter to his face at Antioch when Peter withdrew from eating with Gentile believers (Galatians 2:11–14). The letter has been called the Magna Carta of Christian freedom.
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Dating notes
Dated about AD 48–49, just before the Jerusalem council, if written to the churches of Paul's first journey in southern Galatia; about AD 53–57 if to ethnic Galatians further north. The date is an estimate; Scripture doesn't say when the book was written.