What does Galatians 1:17 mean?

Paul is making the case that the Judaizers are wrong about him. In the following chapters, he will show they are wrong about the truth of the gospel, as well. They are wrong about him, he insists, for suggesting that a) he did not have the authority to teach the Galatians about Jesus and b) he was simply retelling someone else's version of Jesus' message.

Paul has already referred to his dramatic conversion: from Jesus-persecutor to Jesus-follower (Acts 9:1–22). He now relates that after that moment on the Damascus road, he did not go to Jerusalem to receive training about the things of Christ from the 12 disciples, "those who were apostles before me."

Instead of consulting with a human authority, Paul went off by himself to Arabia and then to Damascus for three years (Galatians 1:18). He seems to be saying that it was during this time, after seeing and being commissioned by Christ, that he came to understand the truth of the gospel as he presented it to the Galatians. Since he had not been trained by anyone but Christ Himself, Paul was qualified to fill the role of apostle and his message was trustworthy.