Although the letter was not written by ‘all the brethren’ as well as Paul – for he writes throughout as ‘I’ – yet the brethren knew about the letter and endorsed all that Paul said in it. When he wishes merely to extend greetings from others besides himself, he does so at the end of his epistles.
The use of the plural ‘churches’ here in a letter addressed to a single region shows that Paul recognised each local fellowship as a church in its own right. As such it was complete, having elders and deacons, preaching and ordinances and being a colony of heaven answerable directly to Christ. He does not speak of any human authority over these churches, but only the authority of the word of God as delivered by the apostles and prophets inspired by the Spirit. Each church is therefore ruled directly by Jesus Christ through his word.
The Galatians clearly knew where Paul was when he wrote this epistle, for he sends greetings from the brethren who are with him without stating his location. Galatians 4:20 indicates that, though he would have liked to have done so, he was not near enough to come to them easily and therefore, says Hendriksen, it was probably not sent from Ephesus. Some favour Antioch and others Corinth as Paul’s location.