15 “We who are Jews by birth and not sinful Gentiles 16 know that a person is not justified by the works of the law, but by faith in Jesus Christ. So we, too, have put our faith in Christ Jesus that we may be justified by faith in Christ and not by the works of the law, because by the works of the law no one will be justified.
Paul ragged on favortism earlier in this chapter, but he lets loose with an anti-Gentile barb here. That might be tongue-in-cheek, given that the letter-of-the-law attitude of many Jews of the day left them immunized to the spirit of the law; it's more important to be following Jesus than to get all the menu kosher.
No one's perfect and no one can follow all the rules all the time; Jesus died for all those sins so we didn't have to be pitching a perfect lifetime. We had a perfect game pitched earlier this month, only the 21nd in major league history. If you patched together a century-plus of baseball history and put them on one player, he'd only be two-thirds of the way to a perfect season (your typical starter gets about 35 starts a year).
Perfect seasons are out of reach. Perfect lifetimes are (get out your best Vizzini) inconceivable. That's why Jesus needed to be the sacrifice to end them all.