"None is righteous, no, not one; no one understands; no one sees for God."
Paul said this to the Romans most likely because they believed they were saved because they followed the law. Romans 3:9-20 talks about Jews being under the law and the law revealing their sin. Verse 20 says, "For by works of the law no human being will be justified in His sight, since through the law comes knowledge of sin."
In all of our lives, the law should point out our sins. We should see that we are not justified by the way we live. We can't be. There is too much sin still in our lives. Even as believers we fall and participate in sin. We still do things that, without God's grace, earn us Hell and eternal damnation and separation from God. The law's design is to help hold us accountable to God by giving knowledge of sin. It does not justify us. Only the gift of God's grace can do that.
Praise God for the Law.
Praise God for His Grace.