Adam Clarke on Romans 3:21

But now the righteousness of God - God's method of saving sinners is now shown, by the Gospel, to be through his own mere mercy, by Christ Jesus; without the law - without any right or claim which might result from obedience to the law; and is evidently that which was intended by God from the beginning; for it is witnessed by the law and the prophets - the rites and ceremonies of the one, and the preachings and predictions of the others, all bearing testimony to the great design of God, and to the absolute necessity there was for the sacrifice and salvation which God has provided.