First of all, the style of this epistle to the Galatians is excited and vibrant, which is what the state of the church, which at that time was bewitched by false apostles, demanded. But this epistle has always been a favourite of the great men of the church, and justifiably so.
Jerome was not slow to follow Origen, and after him came Augustine and a host of others. Luther wrote two commentaries on it … which he explained by saying that the greatest danger was that the devil would crush the pure doctrine of faith and bring back false teachings about the merits of works, which sound good and are satisfying to the flesh.
Even when the apostles were still living their churches were not free of such tares. It is impossible to overstate the degree to which human minds are subject to vanity and emptiness, how great is their desire for novelty and how easily people tire of the plain and simple doctrine of the truth. How great and unquenchable too is the hatred of the devil for the gospel and the doctrine of faith, by which he sees his kingdom being destroyed and the rule of Christ, who reigns in our hearts by faith, ushered in instead. …
The apostle himself shows us how necessary the article on true justification is, because if that is left out, Christ disappears, as does Christian freedom and all assurance in life and death, nor is it possible to receive the Holy Spirit of sanctification, who is the source of all good works.
Daniel Toussain, Opera Theologica II, quod Commentarios in Reliquos Divi Pauli Aliorumque Aposolorum Epistolas Continent (Hanau, 1604). Quoted in Gerald L. Bray, Timothy F. George, Scott M. Manetsch (eds), Reformation Commentary on Scripture X: Galatians, Ephesians (Downers Grove IL: IVP Academic, 2011), pp 4-5.