St. Francis de Sales
“Be sure, my daughter, that if you seek to lead a devout life, you must not merely forsake sin, but you must further cleanse your heart from all affections pertaining to sin; for, to say nothing of the danger of a relapse, these wretched affections will perpetually enfeeble your mind, and clog it, so that you will be unable to be diligent, ready, and frequent in good works, wherein nevertheless lies the very essence of all true devotion. Souls which, in spite of having forsaken sin, yet retain such likings and longings, remind us of those persons who, without being actually ill, are pale and sickly, languid in all they do, eating without appetite, sleeping without refreshment, laughing without mirth, dragging themselves about rather than walking briskly. Such souls as I have described lose all the grace of their good deeds, which are probably few and feeble, through their spiritual languor.”
Source: St. Francis de Sales, Introduction to the Devout Life, Random House, Inc., 2002.