One of St. Francis De Sales' most helpful insights is his teaching on the affection for sin. He points out that oftentimes we might turn away from serious sins in our life and try hard not to commit them, but still nurture affection for such sin, which greatly slows down our spiritual progress and disposes us to future falls. He points out that although the Israelites left Egypt in effect, many did not leave it in affection; and the same is true for many of us. We leave sin in effect, but reluctantly, and look back at it fondly, as did Lot's wife when she looked back on the doomed city of Sodom.
Francis gave an amusing but telling example of how a doctor, for the purpose of health, might forbid a patient to eat melons lest he die. The patient therefore abstains from eating them, but "they begrudge giving them up, talk about them, would eat them if they could, want to smell them at least, and envy those who can eat them. In such a way weak, lazy penitents abstain regretfully for a while from sin. They would like very much to commit sins if they could do so without being damned. They speak about sin with a certain petulance and with liking for it and think those who commit sins are at peace with themselves."
Francis says this is like the person who would like to take revenge on someone "if only he could" or a woman who doesn't intend to commit adultery but still wishes to flirt. Such souls are in danger. Besides the real danger of falling into serious sin again, having such a "divided heart" makes the spiritual life wearisome and the "devout" life of prompt, diligent, and frequent response to God's will and inspirations virtually impossible.