"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...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."

Luther, Large Catechism 100-101