What was the truth that the Galatians were not obeying?
The truth was that Jesus Christ was crucified, buried, and risen from the dead, and that salvation is by faith in Him plus nothing. The Galatians were being misled by Judaizers to add works or law-keeping to the gospel, but Paul insisted no addition was required.
Les explains that the Galatians were being hoodwinked by Judaizers from Jerusalem into thinking they had to add to Paul's gospel. They were told it had to be plus something. But the truth was Jesus Christ crucified, buried, and risen from the dead, plus nothing. Les points out that before Paul's gospel, it was always faith plus—Old Testament saints were saved by faith, but not faith alone; they had to repent, be baptized, keep the law, and observe temple worship. Even in Christ's earthly ministry, it was faith plus. But now, beginning with Paul's gospel, it is faith plus nothing. Les illustrates this with a plaque from a lady in his Macalester class, featuring an odd-looking duck with the question 'faith plus nothing?' He notes that most people, 90% of Christendom, say it's faith plus, not faith plus nothing.