Paul fires the cannon-shot across the bow, in case anybody thought that he was doing otherwise - in case people thought that he was somehow instructed by men at some time prior to the writing of this book of Galatians. People could suppose that he knew truth even before he was converted. Paul says, Oh, no, I wasn't instructed by men. I didn't get this from Gamaliel. I didn't get this from the Jews.
Remember I Corinthians 2:10 - these things are revealed through God's Spirit. Now, verse 13 is very revealing.
