If we say we are not saved by keeping the law, but once saved, we must obey it, we have fallen from grace.
Paul’s letter to the Galatians is one of his most polemical, perhaps the most forceful of any. One of the prime things Paul aims to do is dispel the idea that Christians retain any obligation to the law of Moses. In the first chapter, Paul expresses his astonishment at the desertion of the Galatian believers, not just from the message he preached to them, but from God himself. These are the stark terms he uses to summarize the problem. “I am astonished that you are so quickly deserting him who called you in the grace of Christ. The gospel of salvation by Christ alone, apart from any deeds of the law, is the gospel Paul received by revelation of God himself, and which he preached.
Paul’s opponents are largely assumed, because reading Galatians is a bit like hearing one side of a phone conversation. We have to infer what the other person said. But the agitators (as they are usually called) come through at various points by the way Paul answers.
By works of the law, no one will be justified. (2:16)
I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose. 2:21.
For all who rely on works of the law are under a curse; for it is written, “Cursed is everyone who does not abide by all things written in the Book of the Law, and do them.” 3:10.
These are a few statements he makes, demonstrating that his gospel is one of faith in Jesus, wholly apart from the law.
Most evangelicals agree with this, and affirm justification before God is by grace, through faith. The law plays no part in our justification. But even as they agree, some also want to bring the law back in as a way of demonstrating our justification, or as a response of love and thankfulness to God for saving us. Paul will have none of this. Indeed, it sounds reasonable to say that I want to demonstrate my love to God, and how better than by obeying his will in every way, and surely, the law summarizes his will?
These things rest on a priori assumptions, however. Paul elsewhere in the epistle shows that love, not law, is the mark of our obedience to God, and our right response. If I love, I fulfill the law (note, not keep, but fulfill) but if all I do is keep the law, I do not arrive at the place where the gospel delivers me: The new command to love others as Jesus Himself loved us. That is nowhere in the law.
In chapter 6, he even engages in a bit of word play by the phrase “the law of Christ.”
Ronald Fung notes that Paul “speaks of ‘the law of Christ’ polemically, if not almost playfully, as an antithesis to ‘the law of Moses.’ It is as though he said to his converts: if you must observe the law (as the agitators say), do so, only make sure that
the law you observe is not Moses’ law, but the law of Christ.” Rather than commandments associated with the Old Covenant, the law of Christ is instead a principle of self-sacrifice, of loving others as Jesus loved us. This was most clearly demonstrated at the cross.
Secondly, it isn’t stated in Scripture that the law (specifically the “moral law”—the Ten Commandments) represent God’s will for his people today.( If you think I’m saying Christians are thus free to sin as they wish, I invite you to read my other posts on this topic.) In Galatians 3, Paul will contrast Law and Promise, and show that the promise preceded law, and that the law does not nullify promise. The law was added, says Paul, as a temporary thing, with the specific purpose of imprisoning everything under sin, until Christ came. To say that once justified, we now keep the law as a way of showing love to God, is to make Paul say that once the fulfillment of the promise has arrived, we are still under the pedagogue. Yet Paul says the opposite: “So then, the law was our guardian until Christ came, in order that we might be justified by faith. But now that faith has come, we are no longer under a guardian, for in Christ Jesus you are all sons of God, through faith.” 3:24-26. Is Paul saying that once the promise has arrived, the pedagogue still commands believers? By no means. We are no longer under a guardian.
Later in the epistle Paul again makes clear that two covenants cannot coexist at the same time, that is, be in force, any more than both Isaac and Ishmael could both be considered firstborn sons of Abraham. Paul asks of those who want to insist on an obligation to the law, “Tell me, you who desire to be under the law, do you not listen to the law?” 4:21
Here would have been an opportunity for Paul to ensure his hearers understood him properly. That is, if he meant to say that justification is apart from the law, but the law remains their guide for holy living, their standard, he could have said so. But he does not. He says Abraham had two sons, one born of the slave woman (Hagar) and one born of the free woman (Sarah) He explains the two women as two covenants. Any Jew would likely have expected Paul to liken the glorious giving of the law at Sinai with Sarah, but shockingly, he likens the Mosaic Covenant to Hagar! “One is from Mount Sinai, bearing children for slavery” 4:24
Does Paul mean to say that the law (Sinai) can coexist with the the promise? Can the son of the slave woman inherit with the son of the free? “But just as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so also it is now.” 4:29 Here is one of the several places where Paul contrasts flesh and Spirit, and he aligns the law with the flesh. His language here is of contradiction. The flesh persecutes the one born according to the Spirit. They do not happily coexist. Indeed, nowhere in the epistle does he say we, by the Spirit, by faith, are now empowered to keep the law. In 3:12 he has said the law is not of faith. In 3:3, has asks, “Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh?” In other words, you believe you are justified by faith, apart from the law, and do you now think to bring the law back into the Christian life? In 5:1 Paul says “For freedom Christ has set us free; stand firm therefore, and do not submit again to a yoke of slavery.” What is this yoke of slavery if not an obligation to the law?
How does this amount to a “gospel contrary?” It is such a gospel—a distortion as Paul calls it in 1:7—to say that we are justified apart from the law, but that we are obligated to do the things of the law once saved. No equivocation can blunt the force of this. If there is obligation to the law, there is condemnation by the law. It is a backdoor re-introduction of the law in the Christian life, and Paul is adamant: A little leaven leavens the whole lump. We can’t say we are saved by faith, but must live our Christian lives according to the law. That, says Paul, is to fall from grace. It is a gospel contrary to what he preached, to what was revealed to him.
Some believers have had their spiritual lives so formed by law, by an ethos of obligation, that they can’t think of another way. But Paul says “the grace of God training us to renounce ungodliness and worldly passions, and to live self-controlled, upright, and godly lives in the present age.” (Titus 2:12) Grace, not law, is the guide for believers in this age. And it is an entirely sufficient guide. The gospel of grace teaches as all we need to live in manner worthy of the Lord.
 Ronal Y.K. Fung, The Epistle to the Galatians. NICNT. (Grand Rapids, Eerdmans, 1988), 277-278.