The adversaries pretend that faith is only a knowledge of the history of Christ. Therefore, they teach that it can coexist with mortal sin. They say nothing about faith, by which Paul so frequently says that people are justified. For those who are counted as righteous before God do not live in mortal sin. But the faith that justifies is not merely a knowledge of history, it is to believe in God's promise. In the promise, for Christ's sake, forgiveness of sins and justification are freely offered. And so that no one may suppose that this is mere knowledge, we will add further: it is to want and to receive the offered promise of forgiveness of sins and of justification.
The difference between this faith and the righteousness of the Law is easily discerned. Faith is the divine service that receives the benefits offered by God. The righteousness of the Law is the divine service that offers to God our merits. God wants to be worshiped through faith so that we receive from Him those things He promises and offers.
Faith means not only a knowledge of the history, but the kind of faith that believes in the promise. Paul testifies about this in Romans 4:16, "It depends on faith, in order that the promise may rest on grace and be guaranteed." The promise cannot be received unless it comes through faith. Paul connects the promise and faith. (paragraphs 48-50)
Condensed from CONCORDIA: THE LUTHERAN CONFESSIONS, copyright 2005,2006 by Concordia Publishing House. Used by permission. All rights reserved. To purchase a copy of CONCORDIA, call 800-325-3040.