Galatians 3:15

I speak after the manner of men--I take an illustration from a merely human transaction of everyday occurrence. but a man's covenant--whose purpose it is far less important to maintain. if it be confirmed--when once it hath been ratified. no man disannulleth--"none setteth aside," not even the author himself, much less any second party. None does so who acts in common equity. Much less would the righteous God do so. The law is here, by personification, regarded as a second person, distinct from, and subsequent to, the promise of God. The promise is everlasting, and more peculiarly belongs to God. The law is regarded as something extraneous, afterwards introduced, exceptional and temporary (Gal 3:17-19, Gal 3:21-24). addeth--None addeth new conditions "making" the covenant "of none effect" (Gal 3:17). So legal Judaism could make no alteration in the fundamental relation between God and man, already established by the promises to Abraham; it could not add as a new condition the observance of the law, in which case the fulfilment of the promise would be attached to a condition impossible for man to perform. The "covenant" here is one of free grace, a promise afterwards carried into effect in the Gospel.

— Jamieson-Fausset-Brown Commentary (public domain)