Many people not only lose the benefit, but are even the worse for their mortifications [i.e., sacrifices, abstensions], ... because they mistake the whole nature and worth of them: they practice them for their own sakes, as things good in themselves, they think them to be real parts of holiness, and so rest in them and look no further, but grow full of a self-esteem and self-admiration for their own progress in them. This makes them self-sufficient, morose, severe judges of all those that fall short of their mortifications. And thus their self-denials do only that for them which indulgences do for other people: they withstand and hinder the operation of God upon their souls, and instead of being really self-denials, they strengthen and keep up the kingdom of self.

William Law