The plan of salvation was not something new. V21 tells us that the Law and the prophets ‘bear witness to it.” It had existed from the days of eternity.  Imagine how electric it must have been for Paul, after his conversion, to spend 3 years of isolation in Arabia and to have the full glory of the OT dawn on him. Stories he had learned and passages he had memorized since childhood, suddenly all pointing to Christ. From Gen.3:15 to Abraham, (who was declared righteous by faith,) to the most explicit showcase of the plan of salvation in the sacrificial system. It is of interest that John the Baptist referred to Christ as the “Lamb of God, which takes away the sin of the world” (John 1:29) and Paul describes Christ as “our Passover” who was “sacrificed for us’ (1Cor.5:7). 

After his act of adultery and murder, David speaks of “righteousness from God, apart from law” (Rom. 3:21, NIV), when he relies on God’s mercy to “blot out” his “transgressions” and to wash him from his iniquity (Ps.51:1, 2). Isaiah.53 foretells the One who will justify many by bearing their “iniquities” (Isa.53:5). Again, the same chapter notes that “he was wounded for our transgressions, he was bruised for our iniquities: the chastisement of our peace was upon him; and with his stripes we are healed” (v5). 

Jeremiah wasn’t backward on the topic. He refers to the coming Branch of David who would be called “THE LORD OUR RIGHTEOUSNESS” (Jer.23:5, 6). And the great prophetic chapter of Daniel 9 notes: A period of seventy weeks of years^] has been decreed for your people and your holy city to finish their rebellion, to put an end to their sin, to atone for their guilt, to bring in everlasting righteousness, to confirm the prophetic vision, and to anoint the Most Holy one.” (Dan. 9:24NLT). 

Paul knew what he was talking about. God’s plan for saving people by means other than obedience to the law is a theme that runs throughout the OT. It is a theme to which Paul repeatedly turns in his various letters. And it is the theme to which he will now turn as he expounds upon righteousness by faith in Rom.3:21 through 5:21.