Even the Prophets and the Psalms teach free mercy and forgiveness of sins by faith...

Throughout the Prophets and the Psalms this worship (this latreia) is highly praised, even though the Law does not teach the free forgiveness of sins.  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.  To this point belong those frequent repetitions about free mercy and forgiveness of sins by faith that appear in the Psalms and the Prophets.  For example, 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, "I wait for the LORD, my soul waits, and in His word I hope" (v. 5).  This means, "Because You have promised the forgiveness of sins, I am sustained by Your promise."  Therefore, the Fathers also were justified, not by the Law, but by the promise and faith.  It is amazing that the adversaries diminish faith to such a degree, even though they see that it is everywhere praised as a great service.  For example, Psalm 50:15 says, "Call upon Me in the day of troubles; I will deliver you."  God wants Himself to be known.  He wants Himself to be worshipped, so that we receive benefits from Him and receive them because of His mercy, not because of our merits.  This is the richest consolation in all afflictions.  The adversaries band such consolation when they diminish and disparage faith and teach only that by means of works and merits people interact with God.
~BOC, AP, IV (II), 57-60