‘Now this I say. A covenant confirmed beforehand by God, the Law, which came four hundred and thirty years after cannot disannul, so as to make the promise of no effect.'
To put it another way. God, in His unmerited love and favour, gave the inheritance to Abraham and his seed by irrevocable promise. He promised that through them all the nations would be blessed. This was confirmed by God (repeatedly) and, as it were, put on record by Him. The Law came four hundred and thirty years afterwards (see Exodus 12:40 in the Greek Old Testament, the Septuagint). It cannot therefore set aside this irrevocable promise made to Abraham, for such a covenant is irrevocable. And the promise was established long before the Law was given, so its fulfilment cannot depend on fulfilling the Law, for the Law was another, later, covenant made in another context with another man, who also represented a whole, the whole of Israel.
Indeed Moses himself differentiated the two covenants in Deuteronomy 5:3. For he said, ‘Yahweh did not make this covenant (of the ten words at Sinai) with our fathers but with us, even us who are all of us here alive this day'. The covenant of Abraham had reference to the whole world. The covenant of Sinai had reference to the people gathered at the Mount and their seed. It was therefore more exclusive, and not so all-embracing.