God presented him to demonstrate his righteousness at the present time, so that he would be righteous and declare righteous the one who has faith in Jesus.
Romans 3:26 (CSB)

What did God accomplish by revealing his righteousness apart from the law that he gave to his people? As we know, no person is made righteous by the law, for no fallen person is able to obey it perfectly and instead is deserving of God’s judgment (3:19-20). Neither Jews under the Law of God, nor Gentiles, who see the good character of God in creation, have succeeded in obtaining righteousness because all have sinned and fall short of the glory of God (3:23).

Paul helps his readers understand that God’s righteousness through faith in Jesus Christ gives all sinners a way to be right with God. Whether inside or outside of the covenant of the law, all are justified freely by his [God’s] grace through the redemption that is in Christ Jesus (3:24). To be justified is to be proven righteous or declared righteous. How can God justify sinners? Through the atoning sacrifice of Jesus Christ through his blood (3:25). Every sinner who believes in Christ is declared righteous through faith and not by somehow impossibly doing what is morally good.

God has demonstrated his righteousness outside of the law in two key ways. First, he showed that he is just when he passed over sin before the atonement, because he provided Christ as the redeemer for all who believe (3:25). Second, God has showed his righteousness in the present as he declares all who trust in Jesus are justified (3:26). God is undeniably just and the justifier of all who put their faith in Christ.