The Large and Small Catechisms. Martin Luther

no more instruction. For just that is the sin which has been hitherto reckoned among mortal sins, and is called achedia, i.e., torpor or satiety, a malignant, dangerous plague with which the devil bewitches and deceives the hearts of many, that he may surprise us and secretly withdraw God's Word from us.

      For let me tell you this, even though you know it perfectly and be already master in all things, still you are daily in the dominion of the devil, who ceases neither day nor night to steal unawares upon you, to kindle in your heart unbelief and wicked thoughts against the foregoing and all the commandments. Therefore you must always have God's Word in your heart, upon your lips, and in your ears. But where the heart is idle, and the Word does not sound, he breaks in and has done the damage before we are aware. On the other hand, such is the efficacy of the Word, whenever it is seriously contemplated heard, and used, that it is bound never to be without fruit, but always awakens new understanding, pleasure, and devoutness, and produces a pure heart and pure thoughts. For these words are not inoperative or dead, but creative, living words. And even though no other interest or necessity impel us, yet this ought to urge every one thereunto, because thereby the devil is put to flight and driven away, and, besides, this commandment is fulfilled, and (this exercise in the Word) is more pleasing to God than any work of hypocrisy, however brilliant.