1 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: Yet I planted you a fruitful vine, wholly true: how are you turned to bitterness, (and become) the strange vine? 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 man upright, and they have themselves sought out many inventions. For we are His workmanship, says the Apostle, created unto good works, which God afore prepared, that we should walk in them. 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 any one thinks that he is not being burned when sinning, to him the Scripture says, Shall a man wrap up fire in his bosom, and not burn his clothes? For sin burns the sinews of the soul, [and breaks the spiritual bones of the mind, and darkens the light of the heart].
Source: Catechetical Lectures (New Advent)