Presumption follows when a man sets himself to fulfill the Law with works and diligently sees to it that he does what the letter of the Law asks him to do. He serves God, does not swear, honors father and mother, does not kill, does not commit adultery, and the like. Meanwhile, however, he does not observe his heart, does not note the reason why he is leading such a good life. He does not see that he is merely covering the old hypocrite in his heart with such a beautiful life. For, if he looked at himself aright–at his own heart–he would discover that he is doing all these things with dislike and out of compulsion; that he fears hell or seeks heaven, if not also for more insignificant matters: honor, goods, health; and that he is motivated by the fear of shame or harm or diseases. In short, he would have to confess that he would rather lead a different life if the consequence of such a life did not deter him; for he would not do it merely for the sake of the Law. But because he does not see this bad reason, he lives on in security, looks only at the works, not into the heart, and so assumes that he is keeping the Law of God well.
~ Martin Luther, Works, St. Louis edition, 11:81 ff)