(Rom 3:21-22a NET) But now apart from the law the righteousness of God (which is attested by the law and the prophets) has been disclosed – 22 namely, the righteousness of God through the faithfulness of Jesus Christ for all who believe.
The righteousness of God, that is, his faithfulness to his covenant, has been revealed by (get this!) the faithfulness of Jesus. That is, Jesus’ death on the cross is God honoring his covenant. Why?
Well, it all goes back to Gen 15 and the strange blood oath God took as a fire and a smoke pot, passing through a stream of blood from split animals. God made a promise, and Jesus’ death on the cross is God keeping his word. He’ll pay the price. He’ll suffer the consequences.
It’s not so much a vengeful God being assuaged by his kindhearted Son. It’s God himself hanging himself on a cross to pay the price — not only to honor his promise to Abraham but also to do what he asks of us — to pick up our crosses and follow Jesus. And how could Jesus make such a demand if he wasn’t willing to pay the same price?
(Gal 2:15-16 NET) 15 We are Jews by birth and not Gentile sinners, 16 yet we know that no one is justified by the works of the law but by the faithfulness of Jesus Christ [the Messiah, the King]. And we have come to believe in Christ Jesus, so that we may be justified by the faithfulness of Christ and not by the works of the law, because by the works of the law no one will be justified.
We “believe in Christ Jesus,” that is, we have faith in/are faithful to Jesus the Messiah so that we may be justified by Jesus’ faith(fulness). Our faith/faithfulness allows us to be saved by the faithfulness of Jesus and of God.
God is faithful to his promises to Abraham, as shown by the death of Jesus on the cross. And our faith(fulness) allows us to become one with Jesus and so to be enthroned in the heavenly places with Jesus. We are saved by faith — ours and Jesus’ and God’s.