Let me tell you this, even though you know God's Word perfectly and are already a master in all things: you are daily in the devil's kingdom. He ceases neither day or night to sneak up on you and to kindle in your heart unbelief and wicked thoughts against these three commandments (the first three) 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 make a sound, the devil breaks in and has done the damage before we are aware.
Martin Luther, "Large Catechism," 3rd Commandment, paragraph 100