Tertullian

(That) we all know; provided, however, we remember what the same (God) has said through the apostle: "Let your probity appear before men." For what purpose, except that malice may have no access at all to you, or that you may be an example and testimony to the evil? Else, what is (that): "Let your works shine? " Why, moreover, does the Lord call us the light of the world; why has He compared us to a city built upon a mountain; if we do not shine in (the midst of) darkness, and stand eminent amid them who are sunk down? If you hide your lamp beneath a bushel, you must necessarily be left quite in darkness, and be run against by many.