Cardinal Manning states that the third effect of venial sins is that “they dispose the soul for mortal sin. Just as ailments and slight sicknesses are the forerunners which pull down the strength and render men susceptible of greater diseases, so lesser sins prepare the way for greater.”
Take the example of the smouldering temper. “People who are irascible and tempted to anger, though for a long time they fight against it, afterwards begin to indulge it, and to allow the smouldering temper to go on like a charred beam in a house which may smoulder for months before the fire breaks out. Some day there comes an occasion when a temptation meets that smouldering temper, like letting air in on the burning beam; and the whole soul is in a blaze, and malice, or hatred, or resentment, or revenge breaks out into activity.”
Another example can be found in such things as “pattering lies, little insincerities, slight swervings from truth.” Cardinal Manning observes: “The world is full and the atmosphere of the world is thick with those insincerities. They may not be unto death, they may be venial, they may be little lies of courtesy, little falsehoods of excuse; but the day comes when this perverse habit of not speaking the exact truth has so confirmed itself upon the tongue and upon the will, that upon an occasion in which a man would have cut off his right hand rather than have told a lie he will tell a lie boldly and will stand to it. He has been long laying up the fuel for this sin.”
“Venial sins have the effect of giving a perverse inclination to the will. . . . The will which was once united with God, and converted to God, has begun gradually to avert itself from God. There is no such thing as an equilibrium between God and sin; that cannot be; and when the will loses its union with God, it immediately inclines itself towards sin.”
Venial sins are not inconsequential. “They dispose the soul towards greater sin, . . . they keep up the trade of sinning, they blunt the conscience, they bring on insensibility, they cloud the presence of God, they familiarise us with abusing the power which God has given us, against Himself.”
Quotations from Henry Edward Manning, Sin and Its Consequences, 2d ed. (London: Burns and Oates, 1874).