Paul had said the Jews had "much every way" of advantage, chiefly the oracles of God (Romans 3:2). A reader might conclude that Jews stood better before God. Paul asks the question to correct that. "What then?" gathers up the argument; "Are we better than they?" asks whether Jews are in a better position than Gentiles. Barnes explains that the question refers to justification before God, where the Jews had "no preference or advantage over the Gentiles." In the first of two readings he offers, Matthew Henry puts it in the Jew's own voice: "Does this recommend us to God, or will this justify us? No, by no means."

Paul answers at once: "No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin" (Romans 3:9). He then strings together Old Testament verses, beginning, "There is none righteous, no, not one" (Romans 3:10-18). He concludes that "every mouth may be stopped, and all the world may become guilty before God" (Romans 3:19).

"We" here means Paul and his fellow Jews. The verb translated "are we better" is proechometha, which the interlinear glosses "are we better." The phrase "we have before proved" translates proetiasametha, "we have already charged." Barnes points back to the proof in Romans 1:21-32 (the Gentiles) and Romans 2 (the Jews).

To be "under sin" means, as Gill puts it, "under the power and guilt of sin."

This question levels the ground. Israel had real privileges: God's word, His covenants, His promises (Romans 3:2; 9:4). But privileges do not make anyone righteous. Barnes is careful to say the question concerns "the great point in debate, to wit, about justification before God." Paul is not taking back what he said in Romans 3:2.

Once every mouth is stopped (Romans 3:19), Paul can announce the good news: "the righteousness of God without the law is manifested" (Romans 3:21), "unto all and upon all them that believe: for there is no difference" (Romans 3:22). The same "all" who are under sin are offered the same grace.