Voluntary works—besides, over and above God’s commandments—which they call works of supererogation, cannot be taught without arrogancy and impiety. For by them men do declare that they do not only render unto God as much as they are bound to do, but that they do more for his sake than of bounden duty is required; whereas Christ saith plainly: When you have done all that is commanded you, say, We are unprofitable servants.
This is the same in the Episcopal Articles.
Making a Difference in the World 