What was the truth that the Galatians were not obeying in Paul's gospel?
The truth was that salvation rests in Jesus Christ crucified, buried, and risen from the dead, plus nothing. Paul's gospel declares believers are justified by faith alone, without adding law, circumcision, baptism, or any other requirement. The Galatians were being misled by Judaizers to think something must be added to that finished work.
Les explains that the Galatians were being hoodwinked into thinking they had to add to Paul's gospel, which is why he had to hurry and write this letter and called them foolish and bewitched. The truth they should not disobey was Jesus Christ crucified, buried, and risen from the dead, plus nothing, plus nothing. The Judaizers from Jerusalem were saying it had to be plus something. Les contrasts this with what came before Paul's gospel: Old Testament saints were saved by faith, but not faith alone—it was faith plus. Even in Christ's earthly ministry, they had to repent, be baptized, keep the law, and remain under temple worship. But now, beginning with Paul's gospel, it is faith plus nothing. Les notes that 90% of Christendom says it is faith plus, not faith plus nothing.