When 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 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 an atonement for sins. The Holy Fathers often say that we are saved by mercy. When mercy is mentioned, keep in mind that faith, which receives the promise of mercy, is required. Faith justifies and saves, not because it is a worthy work, but only because it receives the promised mercy.
The Old Testament Fathers knew the promise about Christ, that God for Christ's sake wanted to forgive sins. They understood that Christ would be the price for our sins. They knew that our works are not a price for so great a matter. So they received free mercy and forgiveness of sins by faith, just as the saints in the New Testament. [There are] frequent repetitions about mercy and faith that appear in the Psalms and the Prophets. Psalm 130:3 says, "If You, O Lord, should mark iniquities, O Lord, who could stand?" Here David confesses his sins and does not list his merits. He adds, "But with You there is forgiveness" (v. 4). Here he comforts himself by his trust in God's mercy, and he refers to the promise… (paragraphs 53-58)
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.