Two things are generally to be considered, the occasion of this [Galatians] epistle and the scope. The occasion that moved Paul to write this epistle was because certain false apostles slandered him both in respect of his calling as also in respect of his doctrine, teaching that he was no apostle and that his doctrine was false. And by this means they seduced the churches of Galatia, persuading them that justification and salvation was partly by Christ and partly by the law. The scope of the epistle is in three things. First, the apostle defends his calling in the first and second chapters. Secondly, he defends the truth of his doctrine, teaching justification by Christ alone. And upon this occasion he handles the greatest question in the world, namely, what is that justice whereby a sinner stands righteous before God, in the third and fourth and in the beginning of the fifth. Thirdly, he prescribes rules of good life in the fifth and sixth chapters.
William Perkins (1558–1602), Commentary on Galatians, in Works, 2:13 (HT: Inwoo Lee).