When Christ came, the purpose of the Sinai law was achieved. Christ was the fulfillment of the Sinai law. He was its goal, its end, its purpose. By God's design, that Law was preparing Israel for Christ, and through Israel the whole world was being prepared for Christ. But the Jews Paul was writing about retained the Sinai law, and in so doing, they had no room for accepting Christ.
Many people find that too hard to believe.
God doesn't just pretend we are righteous. Surely there is something I must do. Surely I must stop sinning before God will count me righteous. But that is just Paul's point. If we can be righteous ourselves, then we do not need God.