‘And all the brothers who are with me, to the churches of Galatia.'
‘All the brothers who are with me.' Paul joins in with his greeting his fellow-workers, and probably the whole church in the place where he was. He was always aware that he was not just an individual who had been especially blessed by God, but was a part of a whole, and that unity among them was vital (1 Corinthians 3:8). They worked together as one. They were ‘brothers'. And he wanted the Galatians to know that these men and women were one with him in his message. Paul's was not a lone voice. He was supported by the churches in general.
‘To the churches of Galatia.' He is writing to a number of churches within a province. This was probably the Roman province of South Galatia, but the question is only of historic interest. Notice the terse greeting. Nothing is added to soften the description. We may contrast this with 1 Corinthians 1:2 and Ephesians 1:1 where he speaks of those who have been ‘sanctified in Christ' or are ‘faithful in Christ Jesus'. But here he has at present nothing to add, such is his concern about them. The word ekklesia (church, those called out) could be used of groups of Christians in different areas. It could also in other places refer to all Christians seen as one ‘gathering', the ‘congregation' of God's people (Galatians 1:13). It indicated that they were the true people of God, as Israel had been of old. For they are the new Israel, the Israel of God (Galatians 6:16).
Alternately, while sometimes Paul does speak very strongly in this letter, it may be that this initial terseness simply results from his not having yet gained experience in sending Apostolic letters, for Galatians is his first known Apostolic letter.