The very fact that Christ suffered for us, and through His suffering became a propitiation for us, proves that we are (by nature) unrighteous, and that we for whom He became a propitiation, must obtain our righteousness solely from God, now that forgiveness for our sins has been secured by Christ’s atonement. By the fact that God forgives our sins (only) through Christ’s propitiation and so justifieth us by faith, He shows how necessary is His righteousness (for all). There is no one whose sins are not forgiven (in Christ).
Commentary on Romans (Grand Rapids, MI.: Kregel, 1976), 78
(HT: Of First Importance)