Romans 3:21 must be understood within the context of everything that came before it which was a discussion of how God made known to men the reality of sin via the law. Paul then pivots to speak of the revelation of what it means to truly be righteous and appeals to the universal nature of this revelation since He appeared before all men. This ties into Romans 4 because it highlights that the Jews are not the special caretakers of this revelation and their heritage is by faith not the physical descendency from Abraham. All of this is to address the problem in the Roman church which arose from the exile of the Jews from the city causing there to be a Jewish church and a gentile church within Rome rather than having the mixed multitudes of other churches.
I notice you were willing to let The book of James Do that, what about Paul?
Do you have an opinion on what Paul is saying after all these elaborating?