“Don’t make the mistake of thinking that the only righteousness the law required was perfection. It is true that any shortcoming of God’s law offends against his perfect holiness and makes us liable to judgment, since God cannot look with favor on any sin (Habakkuk 1:13; James 2:10-11). But the Old Testament law itself provided for forgiveness and reconciliation for sins. So the requirement of the law was perfection in one sense, but not in the sense that without it a person was lost.
What brought a person to ruin in the Old Testament was not the failure to have the righteousness of sinless perfection. Rather the ruin was caused by the failure to be righteous, first, in the sense that Abraham was “reckoned righteous” by faith in future grace; and, second, in the sense of habitual (thought not perfect) obedience to God which was rooted in an abiding (though not perfect) faith in his future grace. Imperfection would be forgiven, but impenitent, habitual, distrusting disobedience would not.
It is terribly confusing when people say that the only righteousness that has any value is the imputed righteousness of Christ. I agree that justification is not grounded on any of our righteousness, but only on the righteousness of Christ imputed to us. But sometimes people are careless and speak disparagingly of all human righteousness, as if there were no such thing pleased God. They often site Isaiah 64:6 which says our righteousness is as filthy rags. It’s true – gloriously true – that none of God’s people, before or after the cross, would be accepted by an immaculately holy God if the perfect righteousness of Christ were not imputed to us (Romans 5:19; 1 Corinthians 1:30; 2 Corinthians 5:21). But that does not mean that God does not produce in those “justified” people (before and after the cross) an experiential righteousness that is not “filthy rags.” In fact, he does; and this righteousness is precious to God and is required, not as the ground of our justification (which is the righteousness of Christ only), but as an evidence of our being truly justified children of God.”
Quoted from pages 150-151 of John Piper’s book Future Grace.