A fearful thing is sin, and the sorest disease of the soul is transgression, secretly cutting its sinews, and becoming also the cause of eternal fire; an evil of a man’s own choosing, an offspring of the will.

For that we sin of our own free will the Prophet says plainly in a certain place: “I had planted you, a choice vine of fully tested stock; How could you turn out obnoxious to me, a spurious vine?” (Jer 2:21). The planting was good, the fruit coming from the will is evil; and therefore the planter is blameless, but the vine shall be burnt with fire since it was planted for good, and bore fruit unto evil of its own will. For God, according to the Preacher, “made mankind straight, but men have had recourse to many calculations” (Eccl 7:29). “For we are his handiwork, created in Christ Jesus for the good works that God has prepared in advance, that we should live in them” (Eph 2:10). So then the Creator, being good, created for good works; but the creature turned of its own free will to wickedness. Sin then is, as we have said, a fearful evil, but not incurable; fearful for him who clings to it, but easy of cure for him who by repentance puts it from him. For suppose that a man is holding fire in his hand; as long as he holds fast the live coal he is sure to be burned, but should he put away the coal, he would have cast away the flame also with it. If however anyone thinks that he is not being burned when sinning, to him the Scripture says, “Can a man take fire to his bosom, and his garments not burned?” (Prv 6:27). For sin burns the sinews of the soul, and breaks the spiritual bones of the mind, and darkens the light of the heart.

But someone will say, What can sin be? Is it a living thing? Is it an angel? Is it a demon? What is this which works within us? It is not an enemy, O man, that assails you from without, but an evil shoot growing up out of yourself. “Let your eyes look straight ahead and your glance be directly forward” (Prv 4:25), and there is no lust. Keep your own, and seize not the things of others, and robbery has ceased. Remember the Judgment, and neither fornication, nor adultery, nor murder, nor any transgression of the law shall prevail with you. But whenever you forget God, immediately you begin to devise wickedness and to commit iniquity.  

Yet you are not the sole author of the evil, but there is also another most wicked prompter, the devil. He indeed suggests, but does not get the mastery by force over those who do not consent. Therefore says the Preacher, If the spirit of him that has power rise up against you, quit not your place. Shut your door, and put him far from you, and he shall not hurt you. But if you indifferently admit the thought of lust, it strikes root in you by its suggestions, and enthralls your mind, and drags you down into a pit of evils.  

But perhaps you say, I am a believer, and lust does not gain the ascendant over me, even if I think upon it frequently. Do you not know that a root breaks even a rock by long persistence? Admit not the seed, since it will rend your faith asunder: tear out the evil by the root before it blossom, lest from being careless at the beginning thou have afterwards to seek for axes and fire. When your eyes begin to be diseased, get them cured in good time, lest you become blind, and then have to seek the physician.  

God is loving to man, and loving in no small measure. For say not, I have committed fornication and adultery: I have done dreadful things, and not once only, but often: Will He forgive? Will He grant pardon? Hear what the Psalmist says: How great is the multitude of Your goodness, O Lord! And you forgave the wickedness of my heart. TCA  

St. Cyril of Jerusalem (c. 315-386) was bishop of Jerusalem from 350 until his death. He was a vigorous opponent of the heresy of Arianism and is best known for his catechetical lectures of 347 or 348. He was proclaimed a Doctor of the Church in 1882. His feast day is March 18.