Is the Law part of the promises or part of grace?
The Law is not part of the promises, and it is certainly not part of grace. It was added 2,500 years after Adam, 430 years after Abraham in round figures, to serve a purpose, because all the Law could do was condemn Israel and it could not fulfill the promises.
Les explains that if the inheritance were based on the Law, then it would no longer be based on promise. God gave Abram promise after promise—“I will, I will, I will”—and the Law could not fulfill any of those promises because all the Law could do was condemn Israel. The Law was added 2,500 years after Adam, roughly 500 years after Abraham, or 430 years more precisely, so it is not part of the promises and certainly not part of grace. God nevertheless gave the Law for a valid reason.
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.