The words “but now” are found in many places in the Bible, but most often in the Epistles of Paul. These two words are deeply significant, for they indicate a change in program. If my secretary is transcribing some dictation and I say: “But now I would like you to take a letter,” this indicates a change in program.
So it is with this phrase as we find it in Rom. 3:21: “But now the righteousness of God without the law is manifested.” Prior to this time God’s people were under the Law. There was no other way to approach Him. But though under the Law, they constantly broke the Law, so that those who sought salvation by the Law stood before God condemned rather than justified. Thus the Apostle says in Verse 20:
“Therefore by the deeds of the law there shall no flesh be justified in His sight for by the law is the knowledge of sin.”
“But now the righteousness of God without the law is manifested” (Rom. 3:21).
How can this be? How can a man be declared righteous apart from the Law? The answer, the only answer is, by grace through faith in Christ. Though perfect and sinless, Christ died for sin. Whose sin? Yours and mine. Thus as Paul declares in Acts 13:38, 39:
“Through this Man is preached unto you the forgiveness of sins, and by Him all who believe are justified from all things, from which ye could not be justified by the law of Moses.”
“Therefore we conclude that a man is justified by faith, without the deeds of the law” (Rom. 3:28). source