Note 7 at Gal. 3:4: Paul said in this letter that if he preached righteousness by the law, then the offense of the cross would cease (see note at Gal. 5:11). Grace is offensive to religious people. Legalism is not. The suffering that Paul is referring to the Galatians enduring was persecution for preaching the gospel of Christ. If they turned from that gospel to legalism, then all the suffering they had borne for the sake of Christ would be wasted. He is trying to stir them up through the power of memory (2 Pet. 3:1).

Note 8 at Gal. 3:4: Paul is not saying that these Galatians were reprobate (see note 6 at Rom. 1:28, p. 746; see note 1 at Mt. 12:31, p. 186), but he is warning them that they are dangerously close. Everything they had endured for the sake of the gospel would be lost if they renounced their faith in Christ.