" To feel the lusts of the flesh is not without profit to us. It prevents us from being vain and from being puffed up with the wicked opinion of our own work-righteousness. The monks were so inflated with the opinion of their own righteousness, they thought they had so much holiness that they could afford to sell some of it to others, although their own hearts convinced them of unholiness. The Christian feels the unholy condition of his heart, and it makes him feel so low that he cannot trust in his good works. He therefore goes to Christ to find perfect righteousness. This keeps a Christian humble. "

― Martin Luther , Commentary on Galatians