All the Scriptures, both of the Old and New testaments, are divided into, and teach, these two parts, namely, the law and the divine promises. In some places they present to us the law, and in others they offer us grace through the glorious promises of Christ; for example, the Old Testament, when it promises the coming Christ, and through him offers eternal blessings, eternal salvation, righteousness, and eternal life; or the New, when Christ, after his advent, promises in the Gospel, the remission of sins, eternal righteousness and life.
In this place, however, we call the law the Ten Commandments of God, wherever they appear in the Scriptures. It is not our purpose here to speak of the ceremonies and judicial laws.
Now, of these two parts our adversaries choose the law. For since the natural law, which agrees with the law of Moses or the Ten Commandments, is inborn and written in the hearts of all men, and human reason is therefore able, in some measure, to comprehend and understand the Ten Commandments, it imagines that the law is sufficient, and that remission of sin can be obtained through it.
But the Ten Commandments require not only an honorable life, or good works, externally, which reason can to some extent produce; they demand much higher things, beyond all human power and the reach of reason: namely, the law requires us to fear and love God with all sincerity, and from the bottom of our hearts; to call upon him in every time of need, and place our trust in nothing else.
Again, the law demands, that we neither doubt nor waver, but conclude with the utmost certainty in our hearts, that God is with us, hears our prayers, and grants our petitions; it demands, that in the midst of death we expect life and all manner of consolation from God; that in all our troubles we conform entirely to his will; that we shall not flee from him in death and affliction, but be obedient to him, and bear and suffer willingly, whatever may befall us. (This text is from The Henkel Translation of the Book of Concord, which is the only English translation based on the authoritative German Dresden Edition of 1580, p. 157-158).