“But now the righteousness of God has been manifested apart from the law, although the Law and the Prophets bear witness to it—” -Romans 3:21
So now that it has been established that the the works of the law can justify no one—Jew or Gentile—and it can only reveal our sin to us, we are left without any kind of righteousness of our own by which to justify ourselves before God. This is an utterly hopeless condition in which we find ourselves.
Paul says, “but now,” which signifies an adversative position rather than strictly signifying a place in time. In other words, we are not left without hope. There is something else God had in mind, something to which (the teachings of) the Law and the Prophets both testified—the righteousness of God is manifest (made known or revealed) apart from (the works of) the law.
Therefore, it follows that if justification (a righteous standing before God) depends neither on the law itself nor on our own works, it must depend on something else—the mercy of God alone.