Before Paul says anything else — before he even gets to what's wrong — he tells you who he is and where he got his authority from. And he does it in the very first breath.
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Galatians 1:1–2
Paul, an apostle, not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead; and all the brethren which are with me, unto the churches of Galatia.
Notice what he says and what he does not say. He is not an apostle appointed by men. He is not an apostle appointed by man — singular or plural. There is no committee behind him. There is no council that credentialed him. There is no church board that voted him in. His appointment came directly from Jesus Christ and God the Father — the one who raised Christ from the dead.
Now why does he mention the resurrection right there in verse one? Because the resurrection is the proof. It is God's public declaration that everything Christ accomplished was accepted. And the same God who raised the Son from the dead is the one who reached down and grabbed Saul of Tarsus on the road to Damascus and made him an apostle. That is the authority Paul is standing on.
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Then he gives them the salutation in verses 3 through 5. Grace and peace. That is his standard greeting — but watch what he does with it. He does not just say grace and peace from God. He ties it to the work of Christ.
Galatians 1:3–5
Grace be to you and peace from God the Father, and from our Lord Jesus Christ, who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father: to whom be glory for ever and ever. Amen.
He gave himself for our sins. That he might deliver us from this present evil world. According to the will of God. Paul plants the finished work of Christ right at the front door of this letter — before the rebuke, before the defense, before anything. The gospel he is about to defend is the gospel of a Christ who already gave himself, already delivered, and it was already the will of God. Past tense. Done. Complete.
That matters because what the Galatians were being pulled toward was a system that said it was not quite done yet. That there was more to do. That you needed to add something to it. Paul is telling them before he even gets to verse six — the answer to everything that follows is already in the salutation.
MOVEMENT 2: THE SHOCK OF THE DESERTION (1:6–10)
Now here is where Paul does something unusual. In almost every other letter he writes, after the salutation comes a thanksgiving. He thanks God for the people he is writing to. In Romans he thanks God for their faith. In First Corinthians he thanks God for the grace given to them. In Philippians he says he thanks God upon every remembrance of them.
There is no thanksgiving here. None. He goes straight from the salutation to --
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Galatians 1:6
I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel.