Albert Barnes on Galatians 1:11

But I certify you. I make known to you; or, I declare to you. See 1Cor 15:1. Doubtless this had been known to them before, but he now assures them of it, and goes into an extended illustration to show them that he had not received his authority from man to preach the gospel. To state and prove this is the main design of this chapter.

Is not after man. Greek, not according to man. See Gal 1:1. That is, he was not appointed by man, nor had he any human instructor to make known to him what the gospel was. He had neither received it from man, nor had it been debased or adulterated by any human admixtures. He had received it directly from the Lord Jesus.

(*) "certify" "declare to"