What does Galatians 1:20 mean?

Paul interrupts the story of the years after his conversion to Christianity to make a urgent statement to the Galatian believers who have spent time with him. He says, in essence, "As God is my witness, I am not lying!"

Apparently, the Judaizers have been accusing Paul of doing exactly that. It seems they were suggesting that Paul got all his information and training from the apostles and was, therefore, not qualified to be an apostle himself. Paul's story has shown just the opposite. He spent the first years after his conversion—after seeing the Lord and being commissioned as an apostle—off on his own learning about Christ.

He urgently wants the Galatians to understand that he speaks with the authority of one sent out by Jesus and that his teaching to them can be trusted.