Where the heart stands idle and the Word is not heard, the devil breaks in and does his damage

Since so much depends on God’s Word … that nothing is made holy without it, we must realize that God insists upon a strict observance of this commandment and will punish all who despise his Word and refuse to hear and learn it, especially at the times appointed.

Therefore this commandment is violated not only by those who grossly misuse and desecrate the holy day, like those who in their greed or frivolity neglect to hear God’s Word …. but also by that multitude of others who listen to God’s Word as they would to any other entertainment, who only from force of habit go to hear preaching and depart again with as little knowledge of the Word at the end of the year as at the beginning.

Remember, then, that you must be concerned not only about hearing the Word but also about learning and retaining it. Do not regard it as an optional or unimportant matter. It is the commandment of God, and he will require of you an accounting of how you have heard and learned and honored his Word.

In the same way those conceited fellows should be chastised who, after hearing a sermon or two, become sick and tired of it and feel that they know it all and need no more instruction. This is precisely the sin of indolence or satiety — a malignant, pernicious plague with which the devil bewitches and befuddles the hearts of many so that he may take us by surprise and stealthily take the Word of God away from us.
Let me tell you this. Even though you know the Word perfectly and have already mastered everything, still you are daily under the dominion of the devil, who neither day nor night relaxes his effort to steal upon you unawares and to kindle in your heart unbelief and wicked thoughts against all these commandments.

Therefore you must continually keep God’s Word in your heart, on your lips, and in your ears. For where the heart stands idle and the Word is not heard, the devil breaks in and does his damage before we realize it.
On the other hand, when we seriously ponder the Word, hear it, and put it to use, such is its power that it never departs without fruit. It always awakens new understanding, new pleasure, and a new spirit of devotion, and it constantly cleanses the heart and its meditations. For these words are not idle or dead, but effective and living.

Even if no other interest or need drove us to the Word, yet everyone should be spurred on by the realization that in this way the devil is cast out and put to flight, this commandment is fulfilled, and God is more pleased than by any work of hypocrisy, however brilliant.
 
Excerpts from THE THIRD COMMANDMENT – Luther's Large Catechism