Luther, Introduction to The Large Catechism,
Book of Concord:
"9] Therefore I beg such lazy paunches or presumptuous saints to be persuaded and believe for God's sake that they are verily, verily! not so learned or such great doctors as they imagine; and never to presume that they have finished learning this [the parts of the Catechism], or know it well enough in all points, even though they think that they know it ever so well. For though they should know and understand it perfectly (which, however, is impossible in this life), yet there are manifold benefits and fruits still to be obtained, if it be daily read and practised in thought and speech; namely, that the Holy Ghost is present in such reading and repetition and meditation, and bestows ever new and more light and devoutness, so that it is daily relished and appreciated better, as Christ promises, Matt. 18:20: Where two or three are gathered together in My name, there am I in the midst of them.
10] 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 driven away."
"The devil is called the master of a thousand arts. But what shall we call God's Word, which drives away and brings to naught this master of a thousand arts with all his arts and power? It must indeed be the master of more than a hundred thousand arts. 13] And shall we frivolously despise such power, profit, strength, and fruit-we, especially, who claim to be pastors and preachers? If so, we should not only have nothing given us to eat, but be driven out, being baited with dogs, and pelted with dung, because we not only need all this every day as we need our daily bread, but must also daily use it against the daily and unabated attacks and lurking of the devil, the master of a thousand arts."
The First Commandment
"Thou shalt have no other gods before Me.
1] That is: Thou shalt have [and worship] Me alone as thy God. What is the force of this, and how is it to be understood? What does it mean to have a god? or, what is God? 2] Answer: A god means that from which we are to expect all good and to which we are to take refuge in all distress, so that to have a God is nothing else than to trust and believe Him from the [whole] heart; as I have often said that the confidence and faith of the heart alone make both God and an idol. 3] If your faith and trust be right, then is your god also true; and, on the other hand, if your trust be false and wrong, then you have not the true God; for these two belong together, faith and God. That now, I say, upon which you set your heart and put your trust is properly your god.
4] Therefore it is the intent of this commandment to require true faith and trust of the heart which settles upon the only true God, and clings to Him alone. That is as much as to say: "See to it that you let Me alone be your God, and never seek another," i.e.: Whatever you lack of good things, expect it of Me, and look to Me for it, and whenever you suffer misfortune and distress, creep and cling to Me. I, yes, I, will give you enough and help you out of every need; only let not your heart cleave to or rest in any other."
