This question is an understandable response to Paul's stark contrast between the law and the promise (vv. 15-18) and his confinement of the law to a limited role in God's historical plan (vv. 19-20). People who were preoccupied with the supreme value of the law must have been stunned by such a devaluation of it. How could Paul speak against the law? Was the logical conclusion of his line of reasoning the position that the law stood in opposition to the promise? Absolutely not! says Paul. Since both the law and the promise were given by God, they must be complementary rather than contradictory in the overall plan of God. Paul explains the relation of the law to the promise in a two-part answer to the question. First, he presents a contrary-to-fact hypothesis that ascribes a positive role to the law (v. 21). Second, he turns from hypothesis to the reality of the law's negative role (v. 22).

In order to clarify the relation of the law to the promise, Paul poses a contrary-to-fact hypothesis: If a law had been given that could impart life, then righteousness would certainly have come by the law (v. 21). The very way that Paul phrases this hypothesis (as a contrary-to-fact conditional statement) indicates that he does not for a moment think the law can impart life. By life Paul means living in right relationship with God (see 2:19: "that I might live for God"). If the law could empower one to live in a right relationship with God, then righteousness would certainly have come by the law. This was in fact the position of the rival teachers in the Galatian churches. They were promoting the law as the way to live for God. It was actually their position that set the law in direct opposition to the promise; it contradicted the gospel. For as Paul has already said (2:21), "if righteousness could be gained through the law, Christ died for nothing!"

It is only when the law is given a positive role that it is directly opposed to the promise fulfilled in Christ. You are faced with an absolute contradiction if you are told that only by believing in the cross of Christ will you be able to live in a right relationship with God and then you are told that only by keeping the law will you be able to live in a right relationship with God. And that is precisely what the Galatian believers were being told by the rival teachers. But Paul does not accept the false hypothesis of a positive role for the law. Since believing the gospel has already been proved to be the only way to receive life in the Spirit and righteousness (3:1-18), such a positive role for the law is excluded.

The strong adversative conjunction but at the beginning of verse 22 indicates that Paul is turning from the unreal hypothesis of a positive role for the law to the reality of the negative role of the law: but the Scripture declares that the whole world is a prisoner of sin (v. 22). In reality, the law has the negative function of condemning everyone. Literally, Paul says that "the Scripture imprisoned all under sin." Probably Paul has in mind Deuteronomy 27:26, the specific Scripture he quoted in verse 10: "Cursed is everyone who does not continue to do everything written in the Book of the Law." This citation from the law summarizes the purpose of the law: to demonstrate that all are sinners and to put all sinners under God's judgment. Paul's emphasis on the universality of human sin (v. 22) and the universality of God's judgment on all sinners (v. 10) reduces Jews to the same status as Gentiles—the whole world is a prisoner of sin. So identification with the Jewish people by circumcision and observance of the Mosaic law does not remove one from the circle of "Gentile sinners" (2:15) and bring one into the sphere of righteousness, blessing and life. Rather, it leaves one imprisoned under sin.

But we are not left as condemned sinners under the curse of God. The law was given to show that all humanity is held under the bondage of sin, so that what was promised, being given through faith in Jesus Christ, might be given to those who believe (v. 22). Now we can see how the law and the promise work in harmony to fulfill the purpose of God. The law puts us down under the curse; the promise lifts us up in Christ. We are left with no exit under the condemnation of the law so that we might find our freedom only by faith in Christ. The law imprisons all—both Jews and Gentiles—under sin to prepare the way for including all believers in Christ—both Jews and Gentiles—in the blessing promised to Abraham.

So the law should not be viewed as contradictory to the gospel. By reducing all to the level of sinners, the law prepares the way for the gospel. But neither should the law be viewed as if it were the same as the gospel. The law has a negative purpose: it makes us aware of our sin. But it does not, indeed it cannot, set us free from bondage to sin. The promise of blessing comes only through faith in Christ.