Galatians 5:1-6
1 Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage.
2 Behold, I Paul say unto you, that if ye be circumcised, Christ shall profit you nothing.
3 For I testify again to every man that is circumcised, that he is a debtor to do the whole law.
4 Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.
5 For we through the Spirit wait for the hope of righteousness by faith.
6 For in Jesus Christ neither circumcision availeth any thing, nor uncircumcision; but faith which worketh by love.
The Grace New Testament Commentary - Galatians 5:2-4
5:2 Evidently the Judaizers were saying that the men should be circumcised as a commitment to keep the Law. Thus Paul says, "If you become circumcised, Christ will profit you nothing". The future tense could refer to the time immediately following the circumcision, to the Judgment Seat of Christ, or most likely to both.
Circumcision for legalistic reasons is harmful to one's spiritual life. The moment a person adopts legalism, he ceases to benefit spiritually. Only by walking by faith can a Christian find profit in his Christian life.
5:3 In the remainder of this paragraph (vv 3-6), Paul develops the idea that Christ will profit the legalist nothing. Only one who is able to keep the whole law (cf. Galatians 3:10) can be justified by law-keeping. Thus the every man who becomes circumcised in an effort to be justified - which is precisely the reason why the Judaizers were urging circumcision (cf. v 4) - becomes a debtor to keep the whole law. And that is precisely what he cannot do (cf. Acts 15:10).
5:4 Here Paul shifts to the second person: "You have become estranged from Christ". This estrangement does not picture loss of justification, which is impossible. This estrangement is like the break in fellowship that a husband and wife might experience after a major argument. For a believer to attempt to be justified by law results in broken fellowship with Christ. This is true no matter how well intentioned a person may be. Legalism poisons one's motives and hence one's works.
The believer who adopts legalism has "fallen from grace". His fall is from the experience (not position) of grace. Justification by Law is taught by many well-meaning pastors, seminary professors, Sunday school teachers, youth workers, and authors. As a result, the possibility of falling from grace is great today.
Charles Bing - Grace, Salvation, and Discipleship - Falling from Grace, Galatians 5:4
(Salvation) Interpretation: A person who tries to be saved by the law will not be saved.
Second (Salvation) Interpretation: A believer can lose his salvation.
(Discipleship) Interpretation: A believer can choose to ignore the benefits of grace.
The first (Salvation) interpretation assumes the apostle Paul is addressing an unsaved group within the Galatian church. However, we see no indication that Paul is altering his message back and forth between believers and unbelievers. It is clear that he considers the Galatians saved. He reminds them that they had received the Holy Spirit (Galatians 3:1-5), they are sons of Abraham (Galatians 3:26; 4:6-7), they know God (Galatians 4:9), and in the immediate context, they are standing in the liberty of Christ who made them free (Galatians 5:1). To be "estranged from Christ" implies that they were not estranged at one point, which could not describe unbelievers. The phrase "fallen from grace" would be an odd term to indicate unbelief. Rather, it implies that they were in a position of grace, but left it. The context of Paul's discussion in this epistle is the problem of believers who return to the law for either salvation or sanctification.
At least the second (Salvation) interpretation assumes the readers are saved. But the view that they have lost their salvation should be rejected. As seen, Paul appeals to them as Christians. He simply wants the Galatians to live in the benefits of grace, which would be impossible if they returned to the law.
Paul's teaching on grace is the key to this passage and indeed, the whole book of Galatians. The Galatians were called "in the grace of Jesus Christ" (Galatians 1:6). But false teachers were trying to sabotage Paul's teaching and seduce the Galatians to go back under the Mosaic Law (Galatians 1:6-9; 3:1; 4:7; 5:7, 12). Paul writes to show them the folly of trusting in the Law to either save them or sanctify them. Part of his strategy was to show how Peter was being inconsistent with grace (Galatians 2:11-14). In response, Paul affirms, "I do not set aside the grace of God" (Galatians 2:21). Just as the Galatians began in the Spirit, he wants them to continue in the Spirit rather than revert to their own efforts to keep the Law (Galatians 3:2-3). He exhorts them to "stand fast" in their liberty under grace and not go back to the bondage of the Law (Galatians 5:1). They cannot please God or grow under the Law because fleshly efforts do not bring anyone closer to God, nor could they keep the law perfectly.
In verse 4, Paul explains that if the Galatian Christians go back to the law as a means of sanctification, they will become "estranged" (from katargeÅ) from Christ. The word means to be separated or to render something ineffective or powerless. If the Galatians trust in their own efforts to keep the law, they will not be trusting in the power of God's grace through faith. To be "fallen" (from ekpiptÅ) has the idea of losing one's grasp of or drifting from something. They will lose their grip on grace, not their salvation, which is sure and irreversible. Paul is talking about their practice, not their position. They will be operating in contrast to God's grace as a gift to help them grow. They will be cut off from the benefits of His power and provision. Any Christian can live contradictory to who he really is. The thrust of Paul's argument to the Galatians supports the (Discipleship) interpretation.
To be saved by grace as a gift but then try to please God afterward by our own effort is to fall from grace. The grace that saves us is the same grace that keeps us saved and helps us grow. God's grace is everything we don't deserve and more for anything we need, from salvation to ultimate glorification. We can access his grace through faith (Rom. 5:2) for any and every need.