Now, what the righteousness of God is, which is spoken of here [Rom. x.2,3], he immediately afterwards explains by adding: “For Christ is the end of the law for righteousness to every one that believeth.” This righteousness of God, therefore, lies not in the commandment of the law, which excites fear, but in the aid afforded by the grace of Christ, to which alone the fear of the law, as of a schoolmaster, usefully conducts. Now, the man who understands this understands why he is a Christian. For “If righteousness came by the law, then Christ is dead in vain.” If, however, He did not die in vain, in Him only is the ungodly man justified, and to him, on believing in him who justifies the ungodly, faith is reckoned for righteousness. For all men have sinned and come short of the glory of God, being justified freely by His blood. But all those who do think themselves to belong to the “all who have sinned and fall short of the glory of God,” have of course no need to become Christians, because “they that be whole need not a physician, but they that are sick;” whence it is, that He came not to call the righteous, but sinners to repentance.
~ On Nature and Grace chap. 1