All sin, even the inclination to sin. It is the flesh against the spirit, the children of the flesh against the children of the spirit. Sometimes we lose site of that, and we think in terms of mortal sin only, not realizing how important it is to break also the hold that venial sins can have upon us.
In the Catetchism of the Catholic Church, it states, “Venial sin weakens charity; it manifests a disordered affection for created goods; it impedes the soul’s progress in the exercise of the virtues and the practice of the moral good; it merits temporal punishment. Deliberate and unrepented venial sin disposes us little by little to commit mortal sin. However venial sin does not break the covenant with God. With God’s grace it is humanly reparable. ‘Venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity, and consequently eternal happiness.’ [St. Augustine states] “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?” [§1863]