Augustine of Hippo, Against Two Letters of the Pelagians III.20: Why the Righteousness Which is of the Law is Valued Slightly by Paul

Therefore the blessed Paul casts away those past attainments of his righteousness, as “losses” and “dung,” that “he may win Christ and be found in Him, not having his own righteousness, which is of the law.” Wherefore his own, if it is of the law? For that law is the law of God. Who has denied this, save Marcion and Manicheus, and such like pests? Since, then, that is the law of God, he says it is “his own” righteousness “which is of the law;” and this righteousness of his own he would not have, but cast it forth as “dung.” Why so, except because it is this which I have above demonstrated, that those are under the law who, being ignorant of the righteousness of God, and going about to establish their own, are not subject to the righteousness of God? For they think that, by the strength of their own will, they will fulfil the commands of the law; and wrapped up in their pride, they are not converted to assisting grace. Thus the letter killeth them either openly, as being guilty to themselves, by not doing what the law commands; or by thinking that they do it, although they do it not with spiritual love, which is of God. Thus they remain either plainly wicked or deceitfully righteous,—manifestly cut off in open unrighteousness, or foolishly elated in fallacious righteousness. And by this means—marvellous indeed, but yet true—the righteousness of the law is not fulfilled by the righteousness which is in the law, or by the law, but by that which is in the Spirit of grace. Because the righteousness of the law is fulfilled in those, as it is written, who walk not according to the flesh, but according to the Spirit. But, according to the righteousness which is in the law, the apostle says that he was blameless in the flesh, not in the Spirit; and he says that the righteousness which is of the law was his, not God’s. It must be understood, therefore, that the righteousness of the law is not fulfilled according to the righteousness which is in the law or of the law, that is, according to the righteousness of man, but according to the righteousness which is in the Spirit of grace, therefore according to the righteousness of God, that is, which man has from God. Which may be thus more clearly and briefly stated: That the righteousness of the law is not fulfilled when the law commands, and man as it were of his own strength obeys; but when the Spirit aids, and man’s free will, but freed by the grace of God, performs. Therefore the righteousness of the law is to command what is pleasing to God, to forbid what is displeasing; but the righteousness in the law is to obey the letter, and beyond it to seek for no assistance of God for holy living. For when he had said, “Not having my own righteousness, which is of the law, but that which is by the faith of Christ,” he added, “Which is from God.” That, therefore, is itself the righteousness of God, being ignorant of which the proud go about to establish their own; for it is not called the righteousness of God because by it God is righteous, but because man has it from God.