Whence it follows, that the merit of works ceases when righteousness is sought by faith; for it is necessary that this righteousness should be freely given by God, and offered in his word, in order that any one may possess it by faith. To render this more intelligible, when Moses says that faith was imputed to Abram for righteousness, he does not mean that faith was that first cause of righteousness which is called the efficient, but only the formal cause; as if he had said, that Abram was therefore justified, because, relying on the paternal loving-kindness of God, he trusted to His mere goodness, and not to himself, nor to his own merits. For it is especially to be observed, that faith borrows a righteousness elsewhere, of which we, in ourselves, are destitute; otherwise it would be in vain for Paul to set faith in opposition to works, when speaking of the mode of obtaining righteousness. Besides, the mutual relation between the free promise and faith, leaves no doubt upon the subject.
John Calvin, Commentary on the First Book of Moses…, 1.407