Galatians

Galatians is an angry Pauline epistle written to the Church in Galatia (in modern Turkey, the European side of Istanbul) after they abandoned his gospel of salvation by grace alone and embraced the gospel of the Judaizers who said before you could be a good Christian you had to be a good circumcised Jew. His entire message boils down to this line:
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Galatians is one of the epistles that textual analysis confirms Paul actually wrote.