Romans 3:21 But now the righteousness of God has been manifested apart from the law, although the Law and the Prophets bear witness to it
The model of rightness, of what people aught to be, was shown through the Law. It WASN’T the Law, but it was shown through the Law. It was never intended to make people righteous, but it was to show what righteousness was. Now Jesus has come to show what righteousness is. Righteousness is now shown to us as a living, breathing, walking person.
The Law and the prophets bore witness to Jesus’ coming and that the righteousness of God would be revealed in a way that it would actually change people, but it wasn’t in a new set of rules or a new Law. The life that Jesus lived was the righteousness of God. Once He died, the Holy Spirit, the spiritual form of righteousness on display, came to be in all who would believe what Jesus said.
Righteousness is a tricky word. It’s hard to know what it means outside of religious use. Righteousness is like true. If a board isn’t warped, it’s true–it’s righteous. If an instrument is in tune it’s righteous. I saw one dictionary define it as “how it ought to be.” That is pretty good. (That is a righteous definition of righteous!)
Look at Romans 3:21 again in light of that.
Romans 3:21 But now the way things ought to be according to God has been shown and made visible and tangible apart from a list of rules, although all of the rules in the Old Testament and all of the Prophets point to this way things ought to be.