Is the Law part of the promises or grace?
The Law is neither part of the promises nor part of grace. Scripture says the inheritance is not by Law but by promise, and God's covenant with Abraham was based on promise—'I will.' The Law was added later for a purpose; it could not fulfill the promises and only condemned Israel.
Les explains that the Law was not part of the promises and certainly not part of grace. Verse 18 argues that if the inheritance is of the Law, then it is no longer based on promise. God's covenant with Abram was promise, promise, promise—"I will, I will, I will." The Law could not fulfill any of those promises; all it could do was condemn Israel. Verse 19 then asks why the Law was given, and the answer begins, "It was added." Les notes that 2,500 years passed from Adam until Moses received the Law, with about 430 years—roughly 500—between Abraham and the giving of the Law.
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.