Except that is exactly what the Word of God says:
Romans 4:5-8
But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness. Even as David also describeth the blessedness of the man, unto whom God imputeth righteousness without works, Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered. Blessed is the man to whom the Lord will not impute sin.
Romans 4:9-12
Cometh this blessedness then upon the circumcision only, or upon the uncircumcision also? for we say that faith was reckoned to Abraham for righteousness. How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision. And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also: And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised.
Romans 4:21-23
And being fully persuaded that, what he had promised, he was able also to perform. And therefore it was imputed to him for righteousness. Now it was not written for his sake alone, that it was imputed to him;
James 2:23
And the scripture was fulfilled which saith, Abraham believed God, and it was imputed unto him for righteousness: and he was called the Friend of God.
The word imputed, in the Greek is "logizomai", and means:
1) to reckon, count, compute, calculate, count over
a) to take into account, to make an account of
1) metaph. to pass to one's account, to impute
2) a thing is reckoned as or to be something, i.e. as availing for or equivalent to something, as having the like force and weight
b) to number among, reckon with
c) to reckon or account
2) to reckon inward, count up or weigh the reasons, to deliberate
3) by reckoning up all the reasons, to gather or infer
a) to consider, take into account, weigh, meditate on
b) to suppose, deem, judge
c) to determine, purpose, decide
This word deals with reality. If I reckon (logizomai) that my bank book has $25 in it, it has $25 in it. Otherwise I am deceiving myself. This word refers more to fact than supposition or opinion.
So, I would disagree that imputed righteousness would have been a foreign concept to the earliest Christians, seeing as Paul taught it and the concept was also spoken of in the Old Testament as well.
The meaning of these verses is open to debate. The Lutheran and subsequent Protestant contention that Christ’s righteousness covers us like snow on a dunghill derives as much from philosophy as it does to scripture.
The Haydock commentary on Romans says:
Ver. 7. Blessed are they whose iniquities are forgiven, and whose sins are covered. That is, blessed are those who, by doing penance, have obtained pardon and remission of their sins, and also are covered; that is, newly clothed with the habit of grace, and vested with the stole of charity. (Challoner) -— When it is said that the sins of man are covered, we must not imagine that they still remain, but on account of the goodness of God will not be punished, as the Lutherans contend; for the justice of God could not suffer this: but by it we must understand that they are entirely blotted out, and neither exist, nor are considered any longer by God. Still, we must not conclude that man is blessed, as soon as sin is remitted; since the same psalmist, in another place, ascribes happiness to man when he walks in the law of the Lord, and when he keeps judgment and does justice. (Psalms i; cv; and cviii.) And our Saviour says, If you know these things, blessed shall you be if you do them. (St. John xiii.) (Estius) -— Moreover, if sins were never blotted out, but only covered, why did the royal prophet pray to the Almighty, saying: blot out all mine iniquities; and in different parts of the 50th psalm and psalm cviii, speaking of the egregious sinner, he says: let the sin of his mother not be blotted out; which would mean nothing at all, if sins were never blotted out? (Haydock)
Ver. 8. Blessed is the man to whom the Lord hath not imputed sin. That is, blessed is the man who hath retained his baptismal innocence, that no grievous sin can be imputed to him. And likewise, blessed is the man who, after falling into sin hath done penance, and leads a virtuous life by frequenting the sacraments necessary for obtaining the grace to prevent a relapse, that sin is no more imputed to him. (Challoner)
It’s remarkable that you stopped short of citing James 2:24.
As I have been saying all along, there is more to understanding scripture than a surface reading of the text.
St. Paul isn’t sanctioning a dead static faith with God that is once and done.
http://www.newadvent.org/cathen/06701a.htm