God speaking to St Catherine of Siena:

“It is necessary for you to have two lights derived from this primary light, and to these two I will also add a third. The first lightens you all to know the transitory nature of the things of the world, all of which pass like the wind. But this you cannot know thoroughly, unless you first recognize your own fragility, how strong is your inclination, through the law of perversity with which your members are bound, to rebel against Me, your Creator (not that by this law any man can be constrained to commit any, even the smallest sin, against his will, but that this law of perversity fights lustily against the spirit). I did not impose this law upon you, in order that My rational creature should be conquered by it, but in order that he should prove and increase the virtue of his soul, because virtue cannot be proved, except by its contrary. Sensuality is contrary to the spirit, and yet, by means of sensuality, the soul is able to prove the love which she has for Me, her Creator. How does she prove it? When, with anger and displeasure, she rises against herself. This law has also been imposed in order to preserve the soul in true humility. Wherefore thou seest that, while I created the soul to Mine own image and likeness, placing her in such dignity and beauty, I caused her to be accompanied by the vilest of all things, imposing on her the law of perversity, imprisoning her in a body, formed of the vilest substance of the earth, so that, seeing in what her true beauty consisted, she should not raise her head in pride against Me.

The Dialogue of St Catherine of Siena