The Law cannot interfere with the original promise

¹⁵ Let me give you an everyday illustration, my brothers. Once a contract has been properly drawn up and signed, it is honoured by both parties, and can neither be disregarded nor modified by a third party.

^16-18 Now a promise was made to Abraham and to his seed. (Note in passing that the scripture says not “and to seeds” but uses the singular ‘and to your seed’, meaning Christ.) I say then that the Law, which came into existence four hundred and thirty years later, cannot render null and void the original “contract” which God had made, and thus rob the promise of its value. For if the receiving of the promised blessing were now made to depend on the Law, that would amount to a cancellation of the original “contract” which God made with Abraham as a promise.