Everything that is contrary to right reason is sin. Accordingly, therefore, the philosophers think fit to define the most generic passions thus: lust, as desire disobedient to reason; fear, as weakness disobedient to reason; pleasure, as an elation of the spirit disobedient to reason. If, then, disobedience in reference to reason is the generating cause of sin, how shall we escape the conclusion, that obedience to reason—the Word—which we call faith, will of necessity be the efficacious cause of duty? For virtue itself is a state of the soul rendered harmonious by reason in respect to the whole life. Nay, to crown all, philosophy itself is pronounced to be the cultivation of right reason; so that, necessarily, whatever is done through error of reason is transgression, and is rightly called, (ἁμάρτημα) sin. Since, then, the first man sinned and disobeyed God, it is said, “And man became like to the beasts:" being rightly regarded as irrational, he is likened to the beasts. Whence Wisdom says: “The horse for covering; the libidinous and the adulterer is become like to an irrational beast." Wherefore also it is added: “He neighs, whoever may be sitting on him.” The man, it is meant, no longer speaks; for he who transgresses against reason is no longer rational, but an irrational animal, given up to lusts by which he is ridden (as a horse by his rider).
But that which is done right, in obedience to reason, the followers of the Stoics call προσῆκον and καθῆκον, that is, incumbent and fitting. What is fitting is incumbent. And obedience is founded on commands. And these being, as they are, the same as counsel—having truth for their aim, train up to the ultimate goal of aspiration, which is conceived of as the end (τέλος). And the end of piety is eternal rest in God. And the beginning of eternity is our end. The right operation of piety perfects duty by works; whence, according to just reasoning, duties consist in actions, not in sayings. And Christian conduct is the operation of the rational soul in accordance with a correct judgment and aspiration after the truth, which attains its destined end through the body, the soul’s consort and ally. Virtue is a will in conformity to God and Christ in life, rightly adjusted to life everlasting. For the life of Christians, in which we are now trained, is a system of reasonable actions—that is, of those things taught by the Word—an unfailing energy which we have called faith. The system is the commandments of the Lord, which, being divine statutes and spiritual counsels, have been written for ourselves, being adapted for ourselves and our neighbours. Moreover, they turn back on us, as the ball rebounds on him that throws it by the repercussion. Whence also duties are essential for divine discipline, as being enjoined by God, and furnished for our salvation. And since, of those things which are necessary, some relate only to life here, and others, which relate to the blessed life yonder, wing us for flight hence; so, in an analogous manner, of duties, some are ordained with reference to life, others for the blessed life. The commandments issued with respect to natural life are published to the multitude; but those that are suited for living well, and from which eternal life springs, we have to consider, as in a sketch, as we read them out of the Scriptures.