What does it mean that all are under sin in Romans 3:9?
It means both Jews and Gentiles—every person who has ever lived—are guilty before God of disobeying His revealed will, which is sin. Paul declares no ethnic advantage: Jews are not better than Gentiles, for it has been proved all are under sin, both Jews and Gentiles alike.
Les explains that Romans 3:9 must be compared with Genesis 3 and Ezekiel, where the same principle is stated: the soul that sinneth shall surely die. Ezekiel did not disagree with Eve, and Paul does not disagree either. To be “under sin” means every Jew and every non-Jew who has ever lived has been guilty of disobeying the revealed will of God, and that is sin. Paul asks whether Jews are better than Gentiles and answers, “No, in no wise,” because both Jews and Gentiles are all under sin. Les notes this fits with the verses from Romans 3:12 through 20 that follow.
What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin;