The simple answer to this question is that the letter was written because of some serious problems that had arisen in Galatia. Paul never wrote letters without good reason, or, if he did, none such have survived. Normally, his letters were written either in reply to questions received from a church (1 Cor. 7:1), or to disquieting news that he has heard about a church (1 Cor. 1:11), or both. Even a letter like Romans, which at first sight seems to be of a more ‘casual’
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