There is a great irony with divine grace and fallen human nature. The delicacy of divine grace is such that the more you grow in holiness, the less you know it. It is not false humility when St. Francis of Assisi says that he is the greatest of all sinners. Because the more God's light shines within us, more we see what is contrary to it; namely, sin and vice. And because every saint knows where his or her goodness comes from, they do not lay claim to it as their own. Instead, their debt to God is felt all the more. Therefore, if you want a good understanding of sin, go to a saint because with God's light, they can see it for what it is.
Conversely, a veteran sinner— one well experienced in sin —is hardly one to understand sin. Sin is one fact of life in which experience leads to greater ignorance. The more evil a man becomes the less he knows it. Sin is darkness and in darkness one cannot even see oneself and certainly not God.
--Joe Tremblay