In Romans 3:21b, Paul argues that the Old Testament testifies to the work of Christ in manifesting the righteousness of God apart from the law. Paul's intention is to explain that the Old Testament teaches that God's righteousness if given by faith, not by works of the law, and this points to the future work of Christ. Thus, in manifesting the righteousness of God 'apart from the law' Christ accomplishes righteousness for those who have faith. In this way, there is continuity between the Gospel of Christ and the teaching of the Old Testament about achieving righteousness.
Mitchell, James D.
"The Witness of the Old Testament to Christ in Romans 3:21b,"
Diligence: Journal of the Liberty University Online Religion Capstone in Research and Scholarship: Vol. 3
, Article 1.
Available at: https://digitalcommons.liberty.edu/djrc/vol3/iss1/1