So much depends upon God's Word. Without it, no holy day can be sanctified. Therefore, we must know that God insists upon a strict observance of this commandment and will punish all who despise His Word and are not willing to hear and learn it, especially at the time appointed for the purpose.
It is not only the people who greatly misuse and desecrate the holy day who sin against this commandment (those who neglect o hear God's Word because of their greed or frivolity or lie in taverns and are dead drunk like swine). But even that other crowd sins. They listen to God's Word like it was any other trifle and only come to preaching because of custom. They go away again, and at the end of the year they know as little of God's Word as at the beginning. Up to this point the opinion prevailed that you had properly hallowed Sunday when you had heard a Mass or the Gospel read. But no one cared for God's Word, and no one taught it. Now that we have God's Word, we fail to correct the abuse. We allow ourselves to be preached to and admonished, but we do not listen seriously and carefully.
Know, therefore, that you must be concerned not only about hearing, but also about learning and retaining God's Word in memory. Do not think that this is optional for you or of no great importance. Think that it is God's commandment, who will require an account from you about how you have heard, learned, and honored His Word. (Romans 14:12)
Likewise, those fussy spirits are to be rebuked who, after they have heard a sermon or two, find hearing more sermons to be tedious and dull. They think that they know all that well enough and need no more instruction. For that is exactly the sin that was previously counted among mortal sins and is called akadia (apathy or satisfaction). This is a malignant, dangerous plague with which the devil bewitches and deceives the hearts of many so that he may surprise us and secretly take God's Word from us (Matthew 13:19).
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 (Colossians 1:13-14). He ceases neither day nor night to sneak up on you and to kindle in your heart unbelief and wicked thoughts against these three commandments 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 (Matthew 13:24-30) On the other hand, the Word is so effective that whenever it is seriously contemplated, heard, and used, it is bound never to be without fruit (Isaiah 55:11; Mark 4:20). It always awakens new understanding, pleasure, and devoutness and produces a pure heart and pure thoughts (Philippinas 4;8). For these words are not lazy or dead, but are creative, living words (Hebrews 4:12). And even though no other interest or necessity moves us, this truth ought to urge everyone to the Word, because thereby the devil is put to flight and driven away (James 4:7). Besides, this commandment is fulfilled and this exercise in the Word is more pleasing to God than any work of hypocrisy, however brilliant.
(On the third commandment. Luther's Large Catechism).