1 Paul, an apostle (not sent from men nor through the agency of man, but through Jesus Christ and God the Father, who raised Him from the dead), 2 and all the brethren who are with me,
To the churches of Galatia:
Paul is literally on a mission from God. That might be one reason that a lot of folks don't like Paul, for there is a confidence in his writing that comes across as arrogant if you question his calling. Such bravado has the more jaded saying "Oh, yeah?"
He also has a posse with him. A lone fruitcake who thinks God is talking with him is one thing, but Paul has brothers and sisters behind him; that's how the NIV covers "brethren." If they are nuttier than Paul, that gives some credence to the message.