"Wherefore then serveth the law? It was added because of transgressions, till the seed should come to whom the promise was made; and it was ordained by angels in the hand of a mediator."
The question is often asked: To which law is Paul referring? The answer appears as we consider the sole subject of this chapter. Paul is contrasting condemnation and justification, and the chief point of his argument is that "no man is justified by the law in the sight of God." Verse 11. Please take note that the argument is not whether the law operates or not, but whether it operates as a justifier of guilty sinners. Paul clearly spells out in many other texts that the law is necessary as a revealer of sin (Romans 3:20; 7:7), but not as a justifier from sin.
In verse 18 (the verse just preceding the one under consideration), Paul emphasises again that the inheritance is not by law, but by promise. And in verse 21 he says, "If there had been a law given which could have given life, verily righteousness should have been by the law."
These verses make it very clear that Paul is talking about both moral and ceremonial laws in verse 19. Neither of them could save or justify the transgressor. All they could do was condemn the sinner and point forward to "the seed" who "should come." That seed was Christ, and He would be able to justify and deliver them from the condemnation of the law. But even then, the law would not cease to exist. Its function of pointing out sin would ever be needed to turn back to Christ anyone who deviated from the path of justification and obedience.