The following is an excerpt from The MacArthur New Testament Commentary on Galatians 1.

I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel; (1:6)

Paul could hardly comprehend that the Galatian believers were already abandoning his apostolic teaching. He was amazed (thaumazo, a strong word, meaning to be astounded) and bewildered. He could not fathom why they were so quickly deserting Him, that is, God, who called them by means of the grace of Christ. He was not surprised by what the false teachers were doing but was shocked by the favorable response they received from Christians in Galatia.

The apostle was especially surprised that the defection had come so quickly. (For an excellent treatment of the time sequence of Paul’s writings, see Galatians: The Charter of Christian Liberty, by Merrill C. Tenney.) Tacheos (quickly) can mean either easily and readily or soon, and sometimes both, as was probably the case with the Galatians’ deserting the true gospel. The believers apparently offered little and ineffective resistance to the false teachers and therefore were fickle in their allegiance to Paul and his teaching. They quickly and easily came under the influence of heretical doctrines.

The Galatians had been privileged to be taught by the greatest teacher the church has ever known apart from the Lord Himself; yet they readily rejected the truths of grace they had learned from him. There is still a great and urgent need for preaching and teaching that continually repeats the central truths of the gospel (see 2 Pet. 1:12–15). It is possible even for longtime believers to lose a firm grip on those truths and allow themselves to be weakened and perverted by ideas that purportedly improve on the pure and plain teachings of Scripture.

These Galatians were true believers who had come to salvation in the power of the Holy Spirit (3:3, 5; 4:6, 8–9). They were Christian brothers (1:2, 11; 3:15; 4:12, 31; 5:13) who had become seriously confused.

The Galatian Christians not only were being confused and weakened in their confidence to live by grace but were actually deserting. The term behind deserting (metatithemi) was used of military desertion, which was punishable by death during time of war, much as in modern times. The Greek verb is reflexive, indicating that the act is voluntary. The believers were not passively being removed, as the King James translation suggests, but were in the process of removing themselves from the sphere of grace The false teachers were accountable for their corruption of God’s truth, but the Galatian Christians were also accountable for being so easily misled by it to pursue legalism.

To desert the gospel of grace that Paul had taught them was not simply to desert a doctrine but to desert Him, the God who had called them to salvation. Called is an aorist participle and could be translated, “who called you once and for all” (cf. 2 Thess. 2:13–14; 2 Tim. 1:8–9; 1 Pet. 1:15). The call spoken of in the New Testament epistles is always an effectual call to salvation (see Rom. 8:30).

The only gospel of God is the gospel of grace, which is the gospel of divine redemption totally apart from any work or merit of man. “For by grace you have been saved through faith,” Paul declared to the Ephesians, “and that not of yourselves, it is the gift of God; not as a result of works, that no one should boast. For we are His workmanship” (Eph. 2:8–10). And it is continually that “grace in which we stand” (Rom. 5:2). We live in grace from the moment of salvation, and if grace ever stopped, we would lose our undeserved salvation and perish in sin. The grace of Christ is God’s free and sovereign act of love and mercy in granting salvation through the death and resurrection of Jesus, apart from anything men are or can do, and of His sustaining that salvation to glorification. It is absurd to accept a gracious salvation and then endeavor to maintain righteousness through human efforts, ceremonies, and ritual.

The Judaizers who plagued the early church claimed to be Christians, and much of their doctrine was orthodox. They must have recognized Jesus as the promised Messiah and even acknowledged the value of His sacrificial death on the cross-otherwise they would never have gotten a hearing in the church. They claimed to believe all the truths that other Christians believed. They did not purport to overtly deny the gospel but to improve it by adding the requirements, ceremonies, and standards of the Old Covenant to the New. But anything added to grace destroys it just as surely as does anything taken from it. When law-even God’s own law-is added to His grace, His grace ceases to be grace (cf. Rom. 11:6).

The most destructive dangers to the church have never been atheism, pagan religions, or cults that openly deny Scripture, but rather supposedly Christian movements that accept so much biblical truth that their unscriptural doctrines seem relatively insignificant and harmless. But a single drop of poison in a large container can make all the water lethal. And a single false idea that in any way undercuts God’s grace poisons the whole system of belief.

Paul would not tolerate a single drop of legalism being intermixed with God’s pure grace. To turn away from any part of the grace of Christ is to turn away from the power of God to that of human effort. Those who seek to sustain their justification in any degree by law have “fallen from grace” and “have been severed from Christ” (Gal. 5:4). Paul is not speaking of losing salvation that is already received but of polluting the pure stream of living grace by putting a barrier between oneself and Christ and therefore of being severed, or separated, from His power and from fellowship with Him. It is impossible to forsake grace without forsaking the Lord, so Paul called Timothy to “be strong in the grace that is in Christ Jesus” (2 Tim. 2:1) and testified to his own life in grace in 1 Corinthians 15:10.

The Judaizers were promoting a different gospel, a completely contrary and ineffective means of being right with God. Consequently, although they had “begun by the Spirit,” some of the true believers in Galatia were trying to be “perfected by the flesh” (3:3). Although they had “come to know God, or rather to be known by God,” they had turned “back again to the weak and worthless elemental things” (4:9); and although they had been “running well,” they were now being “hindered … from obeying the truth” (5:7).