In order to show mans need of a righteousness from God, Paul begins by demonstrating that all men are sinners
Paul demonstrates that:
Gentiles are sinful and without excuse (Romans -32)
Jews are sinful and without excuse (Romans 2:1-3:8)
Paul then concludes that all men are sinners (Romans3:9-20)
Romans 3:9 - What shall we conclude then? Are we [Jews] any better [than the Gentiles]? Not at all! We have already made the charge [in Romans -3:8] that Jews and Gentiles alike are all under sin.