Martin Luther's Large Catechism

Besides, it is an exceedingly effectual help against the devil, the world, and the flesh and all evil thoughts to be occupied with the Word of God, and to speak of it, and meditate upon it, so that the First Psalm declares those blessed who meditate upon the law of God day and night. Undoubtedly, you will not start a stronger incense or other fumigation against the devil than by being engaged upon God's commandments and words, and speaking, singing, or thinking of them. For this is indeed the true holy water and holy sign from which he flees, and by which he may be driv