Cassiodorus

When he calls himself an apostle not of human making but through Christ Jesus, he does away with those who had only human authority for styling themselves apostles. The churches at that time were being thrown into turmoil by false preachers. He greets these churches with all the brethren who are with him. In that greeting he also blesses them, so that their fitness to receive the word of the Lord may be established. SUMMARY OF GALATIANS 1.1.1.Paul Speaks with Passion. CHRYSOSTOM: [The first verse] is full of great passion and strong sentiment; and not the prologue only but, as it were, the whole letter. For always to speak mildly to those who are being taught, even when they need vehemence, is not the part of a teacher but of a corrupter and an enemy.

He says this so that Hebrews no less than Gentiles may be compelled to accept the grace of faith, not the impositions of the law, which no one could fulfill.