Whenever we speak of justifying faith, we must keep in mind that these three objects belong together: the promise, grace, and Christ’s merits as the price and atonement. The promise is received through faith. Grace excludes our merits and means that the benefit is offered only through mercy. Christ’s merits are the price, because there must be a certain atonement for our sins. Scripture frequently cries out for mercy; the Holy Fathers often say that we are saved by mercy. Therefore, whenever mercy is mentioned, we must keep in mind that faith, which receives the promise of mercy, is required there. Again, whenever we speak about faith, we want an object of faith to be understood, namely, the promised mercy. For faith justifies and saves, not because it is a worthy work in itself, but only because it receives the promised mercy.
Concordia : The Lutheran Confessions, Edited by Paul Timothy McCain, 89 (St. Louis, MO: Concordia Publishing House, 2005).