And all the brethren which are with me - It was usual for Paul to associate with him the ministers of the gospel, or other Christians who were with him, in expressing friendly salutations to the churches to which he wrote, or as uniting with him, and concurring in the sentiments which he expressed. Though Paul claimed to be inspired, yet it would do much to conciliate favor for what he advanced, if others also concurred with what he said, and especially if they were known to the churches to which the epistles were written. Sometimes the names of others were associated with his in the Epistle; see the 1 Corinthians 1:1 note; Philippians 1:1 note; Colossians 1:1 note; 1 Thessalonians 1:1 note. Since we do not know where this epistle was written, of course we are ignorant who the “brethren” were, who are here referred to. They may have been ministers with Paul, or they may have been the private members of the churches. Commentators have been much divided in opinion on the subject; but all is conjecture. It is obviously impossible to determine.
Unto the churches - How many churches there were in Galatia is unknown. There were several cities in Galatia, as Ancyria, Tavia, Pessinus, etc. It is not improbable that a church had been established in each of the cities, and, since they were not far distant from each other, and the people had the same general character and habits, it is not improbable that they had fallen into the same errors. Hence, the Epistle is directed to them in common.