The law of Moses was given through angels and a mediator (Moses). The law required agreement between God and the people. The law was conditional, with blessings for obedience and curses for disobedience. In contrast, God gave his promise directly to Abraham without a mediator. It was God's promise alone, and God did not require anything in return.
but God is one: Paul used the basic Jewish creed (see Deuteronomy 6:4) to persuade non-Jewish Christians not to convert to Judaism.