The editor of Light and Peace quotes St. Francis de Sales extensively on various aspects of conversion from sin:
St. Francis says: “God has, indeed, on some occasions cured sinners instantaneously and without leaving in them any trace of their previous maladies. Such, for instance, was the case with the Magdalen. In a moment her soul was changed from a sink of corruption into a well-spring of perfection, never again to be contaminated by sin. But, on the other hand, in several of the beloved disciples this same God allowed many marks of their evil inclinations to remain for some time after their conversion, and this for their greater good. Witness Saint Peter, who, even after the divine call, was guilty of various imperfections and once fell totally and miserably by the triple denial of his Lord and Master.”
“Solomon says (Prov 30:23) there is no one more insolent than a servant who has suddenly become mistress. A soul that after a long slavery to its passions should in a moment subjugate them completely, would be in great danger of becoming a prey to pride and vanity. This dominion must be gained little by little, step by step; it cost the saints long years of labor to acquire it. Hence the necessity of having patience with every one, but first of all with yourself.”
“There is no sight more pleasing to Heaven than to witness the persevering and determined struggle of a soul which, throughout, remains united to God by a sincere desire and a firm resolution not to offend him—and maintaining this struggle calmly and patiently even when it is to all appearance fruitless. Such a soul, resigned to retain its defects if it is God’s will, yet determined notwithstanding to fight against them relentlessly, is more precious in the eyes of God than if the practice of virtue were easy for it and it were in peaceful possession of spiritual gifts. Labor, then, in the presence of your heavenly Father; struggle on with strength and courage; but do not be too desirous of success, for when this craving for self-satisfaction is excessive it is sure to be accompanied by vexation and impatience.”
“We should labor, therefore, without any uneasiness as to results. God requires efforts on our part, but not success.”
Quotations from Carlo Giuseppe Quadrupani, Light and Peace: Instructions for Devout Souls to Dispel Their Doubts and Allay Their Fears (St. Louis: B. Herder, 1898).