Romans 3:20

This is because no human being can be considered right with God by [obeying] the requirements of a law. [Note: It is difficult to determine the exact sense in which "law" is used in these verses. It may be law generally, or the law of Moses specifically. See Bruce, pages 52-58]. For the knowledge of what sin is comes from the law of Moses.

Treasury of Scripture Knowledge

Summary

I know it is so of a truth: but how should man be just with God?

Bible References

How

Romans 3:20
This is because no human being can be considered right with God by [obeying] the requirements of a law. [Note: It is difficult to determine the exact sense in which "law" is used in these verses. It may be law generally, or the law of Moses specifically. See Bruce, pages 52-58]. For the knowledge of what sin is comes from the law of Moses.