I'm not making this stuff up!

That is what Paul seems to be saying over and over in the beginning of Galatians, "I'm really not making this stuff up!"

Paul, an apostle, (not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead;) (Galatians 1:1; ESV)

For I did not receive it from any man, nor was I taught it, but I received it through a revelation of Jesus Christ. (Galatians 1:12; ESV)

As evidence of this, take the following:

• Compare the early gospel message of Paul to that of the apostles. He did not confer with them to get his story straight. The apostles added nothing to Paul's gospel (Gal 2:6). Paul was converted, the truth was revealed to him, and he went directly to preaching the gospel (Gal 1:15-17), which turned out to be the same gospel that was imparted to the apostles. Later, when they saw him and heard his message they recognized its validity (Gal 2:9).
• This message was not taught to him by men. This is not the type of thing that men teach, and certainly was not what he had been taught as a Jew (Gal 1:13-14, Gal 2:19). It is as if he is saying, "This message is so unlike the type of things men make up, I must either be crazy or truthful." And from the clarity of Paul's rhetoric you certainly can't think he is crazy or deluded.