God’s promise to Abraham was regarding a son and his inheritance. “For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise.” Your honest question then, “Why the Law then? It was added because of transgressions, having been ordained through angels by the agency of a mediator, until the seed would come to whom the promise had been made. Now a mediator is not for one party only; whereas God is only one” (Gal 3:18-20). Your spiritual inheritance is based upon God’s grace through His promised salvation through the finished work of Jesus Christ and not upon your performance of the Law and good works.