Jamieson, Fausset and Brown on Deuteronomy 5:3

3. The Lord made not this covenant with our fathers, but with us--The meaning is, "not with our fathers" only, "but with us" also, assuming it to be "a covenant" of grace. It may mean "not with our fathers" at all, if the reference is to the peculiar establishment of the covenant of Sinai; a law was not given to them as to us, nor was the covenant ratified in the same public manner and by the same solemn sanctions. Or, finally, the meaning may be "not with our fathers" who died in the wilderness, in consequence of their rebellion, and to whom God did not give the rewards promised only to the faithful; but "with us," who alone, strictly speaking, shall enjoy the benefits of this covenant by entering on the possession of the promised land.