If people of the law were heirs, what would happen to faith and the promise?

If those who are of the law were heirs, faith would be made void and the promise would be made of none effect. The law works wrath, so it cannot bring anyone to heaven. Abraham's heirship came through the righteousness of faith, not through the law or Mosaic legal system.

Les explains that Paul, by inspiration, points to Abraham as the perfect example of one justified by faith alone, plus nothing. The promise that Abraham would be heir of the world was not given through the law but through the righteousness of faith. Therefore, if those who are of the law were heirs, faith would be void and the promise of none effect, because the law works wrath. Les notes this is hard for people to swallow, since many think commandment-keeping will get them to heaven, but the commandments only bring the wrath of God. He also stresses that grace is not license.

Romans 4:14

For if they which are of the law be heirs, faith is made void, and the promise made of none effect:

See the rest

What happens if keeping the law makes people heirs of the promise?

If those who are of the law are heirs, then faith is made void and the promise is made of none effect, because the law works wrath. The promise to Abraham was not through the law but through the righteousness of faith.