5 Effects of Venial Sin from St. Ignatius

While he is in the flesh, man cannot help but have at least some light sins. But do not despise these sins which we call “light”: if you take them for light when you weigh them, tremble when you count them. A number of light objects makes a great mass; a number of drops fills a river; a number of grains makes a heap. What then is our hope? Above all, confession. – St. Augustine

The following is an excerpt from The Spiritual Exercises of St. Ignatius Loyola.

Venial sin, it is true, does not destroy in us habitual grace; but, nevertheless, how deplorable are its effects in the soul!

1.  It imprints a stain which tarnishes its beauty. It is to the soul what an ulcer is to the body.
2.  It weakens the lights of the spirit and the fervour of the will; and from that arise langour in prayer, in the use of the Sacraments, and in the practice of Christian virtues.
3. It deprives the soul of the super-abundance of graces — choice graces, which God only gives to the purity of heart.
4. It deprives the soul of a greater degree of grace and glory which it would have acquired by its fidelity, and which is lost by its fault. A God less glorified eternally, less loved, and less possessed, — such are the consequences of venial sin to the soul.
5. It leads to mortal sin as sickness leads to death; for the repetition of venial sins insensibly weakens the fear of God, harden the conscience, forms evil attachments and habits, gives fresh strength to the temptations of the enemy of our salvation, nourishes and develops the passions. Hence the Holy Spirit says, “He that contemneth small things, shall fall by little and little” (Ecclus. xix. 1); and that of our Saviour, “He that is unjust in that which is little, is unjust also in that which is greater” (Luke xvi. 10).

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