Galatians 1:2

all the brethren--I am not alone in my doctrine; all my colleagues in the Gospel work, travelling with me (Act 19:29, Gaius and Aristarchus at Ephesus: Act 20:4, Sopater, Secundus, Timotheus, Tychicus, Trophimus, some, or all of these), join with me. Not that these were joint authors with Paul of the Epistle: but joined him in the sentiments and salutations. The phrase, "all the brethren," accords with a date when he had many travelling companions, he and they having to bear jointly the collection to Jerusalem [CONYBEARE and HOWSON]. the churches--Pessinus and Ancyra were the principal cities; but doubtless there were many other churches in Galatia (Act 18:23; Co1 16:1). He does not attach any honorable title to the churches here, as elsewhere, being displeased at their Judaizing. See First Corinthians; First Thessalonians, &c. The first Epistle of Peter is addressed to Jewish Christians sojourning in Galatia (Pe1 1:1), among other places mentioned. It is interesting thus to find the apostle of the circumcision, as well as the apostle of the uncircumcision, once at issue (Gal 2:7-15), co-operating to build up the same churches.

— Jamieson-Fausset-Brown Commentary (public domain)