Thomas Aquinas

“ Further, if sin causes a stain, this stain is either something positive, or a pure privation. If it be something positive, it can only be either a disposition or a habit: for it seems that nothing else can be caused by an act. But it is neither disposition nor habit: for it happens that a stain remains even after the removal of a disposition or habit; for instance, in a man who after committing a mortal sin of prodigality, is so changed as to fall into a sin of the opposite vice. Therefore the stain does not denote anything positive in the soul. ”

“ Therefore a stain is the effect of sin.
I answer that, A stain is properly ascribed to corporeal things, when a comely body loses its comeliness through contact with another body, e.g. a garment, gold or silver, or the like. Accordingly a stain is ascribed to spiritual things in like manner. Now man's soul has a twofold comeliness; one from the refulgence of the natural light of reason, whereby he is directed in his actions; the other, from the refulgence of the Divine light, viz. of wisdom and grace, whereby man is also perfected for the purpose of doing good and fitting actions. ”

“ For although the act of sin ceases, whereby man withdrew from the light of reason and of the Divine law, man does not at once return to the state in which he was before, and it is necessary that his will should have a movement contrary to the previous movement. ”