The order of this epistle is fitting in that, after the two epistles to the Corinthians, in the first of which it is a question of the sacraments of the Church, and in the second, of the ministers of these sacraments, there should necessarily follow the epistle to the Galatians, treating of the termination of the sacraments of the Old Testament. (Commentary on Galatians, I.1)

This is interesting for two reason:  First, most scholars today recognize Galatians to be the earliest of Paul’s epistles.  Second, he sees the main issue in this epistle as the problems of passing from one age to the next – this is also the opinion of Augustine – which I believe is the correct perception.