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.

Les explains that Paul, by inspiration, goes back to Abraham as the perfect example of being justified by faith alone, plus nothing. The promise that Abraham would be heir of the world was not through the law but through the righteousness of faith. If those who are of the law were heirs, then faith is made void and the promise is made of none effect, because the law worketh wrath. Les emphasizes that the commandments can only bring the wrath of God, not heaven, and 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:

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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.