Humanity is sinful, but the Book of Romans explains how faith in Jesus justifies Jews and gentiles and creates a multiethnic covenant family that's unified through love and forgiveness.
The people of Israel tried to obey God and follow the Law, but they didn't succeed. Only faith in Jesus' death and resurrection can justify humanity and fulfill God's promise to create a covenant relationship with His people, the descendants of Abraham.
Through Jesus, God creates a new covenant family that includes Jews and gentiles who are unified as they love and forgive each other. Even though people continue to reject Jesus, God uses their rejection to expand His family and grow the Church.