Paul said that there were advantages in being Jewish. Having the Word of God was the primary one.
Romans 3:9-20 (KJV)
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;
As it is written, There is none righteous, no, not one:
There is none that understandeth, there is none that seeketh after God.
They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one.
Their throat is an open sepulchre; with their tongues they have used deceit; the poison of asps is under their lips:
Whose mouth is full of cursing and bitterness:
Their feet are swift to shed blood:
Destruction and misery are in their ways:
And the way of peace have they not known:
There is no fear of God before their eyes.
Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God.
Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
Paul asked the next logical question. Did that mean that the Jews were better than the Gentiles (verse 9)? His answer in the KJV is, "No, in no wise." Other translations use, "No, not at all." However, the Greek is ou pantos, ou meaning "no" or "not" and pantos meaning "entirely, wholly, certainly." Together, as David Stern points out in the Jewish New Testament Commentary, (Jewish New Testament Publications, Inc, Clarksville, Maryland, 1992, p. 341-342) the literal meaning seems to be "not entirely." Paul was acknowledging that because the Jews had the Word of God, they had the advantage over the Gentiles and should have been better. However, as Paul had been proving, both Jews and Gentiles were on equal footing when it came to sin.
Paul then began a list of the common ground that the Jews and Gentiles had regarding sin. He pulled his list from the Old Testament where, "it is written." No one is righteous (Psalm 14:3) (verse 10). No one understands and no one seeks God (Psalm 14:2 and 53:2) (verse 11). They are all gone out of the way, they have become unprofitable, and none do good (Psalm 14:3) (verse 12). They speak deceitfully and in other evil ways (Psalm 5:9) (verse 13). They speak in curses and bitterness (Psalm 10:7) (verse 14). They shed blood (verse 15). Their ways lead to destruction and misery (verse 16). Their ways are not peaceful (Isaiah 57:21) (verse 17). They have no fear of God (Proverbs 1:17, Psalm 36:1, Ecclesiastes 12:13) (verse 18).
Again, Paul was painting a very dark picture of mankind. Although he understood that man is capable of doing good deeds, our bent is to do otherwise. There is no one who is really righteous by nature.
In verse 19 Paul said that whatever the law says (and he is referring to those comments taken from the Old Testament in verses 10 through 18) is said to the Jews. So, if the Jews, with the advantage of the Word of God, were guilty before God, the Gentiles, without that guidance of the law, were also guilty (Paul proved this already in 1:18-2:16).
Paul summarized in verse 20. The Jews often claimed superiority over the Gentiles, when they should have understood how sinful man really is, since they knew the law and saw how both groups missed the mark. Obviously, no one could claim justification by their deeds.
An aside: After reading this section of Romans, I was struck by some of the wisdom that is expressed in Christianity today. I've often seen comments like, "I'm not perfect, but God loves me anyway." Although this statement is true in a very generic sense, the tone implies that God doesn't care about how much we sin, or even approves of our sin so that our salvation from such sin is magnified. Paul's condemnation of mankind, both Jew and Gentile, should cause us to look deep within ourselves to see the evil that lurks there and be determined to allow the Holy Spirit to remove any and all sin found within.
What did Paul say next? Was there more condemnation, or was he ready to give his readers some good news?