Satan is one of these. Paul knew that he too was not to be the standard of right. The gospel was true because it was the gospel of God. Paul was willing to stir up controversy over the gospel. Controversy is right when the gospel is at stake.
Congregations are responsible for discerning the truth.
In doing this, he appealed to them to judge their teachers' teaching. This letter seems to assume that the gospel is clear to Christians and churches. We are to judge everything by the Word of God. If it is different from what we have in the Bible, we have the responsibility to reject it. We need to be willing to enter controversy when the message of the gospel of Christ is at stake.