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”.
— The Instructor I.XIII
“Now my just one shall live by faith,” the prophet said. “Except ye believe, neither shall ye understand.” For how ever could the soul admit the transcendental contemplation of such themes, while unbelief respecting what was to be learned struggled within? But faith, which the Greeks disparage, deeming it futile and barbarous, is a voluntary preconception, the assent of piety—“the subject of things hoped for, the evidence of things not seen,” according to the divine apostle. “For hereby,” preeminently, “the elders obtained a good report. But without faith it is impossible to please God.” Others have defined faith to be a uniting assent to an unseen object, as certainly the proof of an unknown thing is an evident assent. If then it be choice, being desirous of something, the desire is in this instance intellectual. And since choice is the beginning of action, faith is discovered to be the beginning of action, being the foundation of rational choice in the case of any one who exhibits to himself the previous demonstration through faith. Voluntarily to follow what is useful, is the first principle of understanding. Unswerving choice, then, gives considerable momentum in the direction of knowledge. The exercise of faith directly becomes knowledge, reposing on a sure foundation. Knowledge, accordingly, is defined by the sons of the philosophers as a habit, which cannot be overthrown by reason. Is there any other true condition such as this, except piety, of which alone the Word is teacher? I think not. Theophrastus says that sensation is the root of faith. For from it the rudimentary principles extend to the reason that is in us, and the understanding. He who believeth then the divine Scriptures with sure judgment, receives in the voice of God, who bestowed the Scripture, a demonstration that cannot be impugned. Faith, then, is not established by demonstration. “Blessed therefore those who, not having seen, yet have believed.”
— The Stromata II.II