Cardinal Manning explains that, although venial sins “do not break our friendship with God,” they are not without harmful consequences.
“First, venial sins diminish the grace of God in the soul. When theologians say that venial sins diminish grace, they always make this distinction—they do not mean to say that the quantity of the grace of God is made less, because the grace of God is like life, which cannot be diminished. We are either alive or dead; but the living powers may be diminished. Life remains, but the health and the vigour and the strength of the living man are lessened.” And “the fervour and the operation and energy and efficacy of grace” is diminished.
“St. Bernard says that fervour—that is to say, the life of fidelity and obedience—has many effects; and two of those effects are these. First, it renders whatever we have to do easy to us; and secondly, whatever we do easily, we do with pleasure, and find a sweetness in it. They know this who have learned to speak a foreign language, or to use a musical instrument. Nothing is more tedious, repulsive, or trying than the acquisition either of a foreign language or of the practice of music; but the moment we have attained a certain facility in either, there is a sweetness in exercising that acquired skill.”
“Fervour consists in these three things: regularity, punctuality, and exactness—that is, doing our duty to God by rule; doing it punctually at the right time; and exactly, that is, as perfectly as we can. But if we have been indulging venial sins of any sort or kind, we begin to do our duty towards God in a slovenly way; we neglect the right time; we do it irregularly; we put God off with an imperfect service.”
“Venial sins are like the dust settling upon the perfect machine of which I spoke. As the dust accumulates upon the timepiece, the motion of the timepiece becomes slower; and as it becomes sluggish it loses its perfection.”
If mortal sin is “the death of the soul,” then venial sin is “the disease of the soul,” a lingering spiritual illness which settles upon “those who willingly allow themselves to fall into such infirmities and imperfections.”
Quotations from Henry Edward Manning, Sin and Its Consequences, 2d ed. (London: Burns and Oates, 1874).