Credited to him as righteousness:
Protestant teaching in a nutshell: Justification comes about by the imputation of Christ’s Righteousness to the sinner’s account (though the sinner’s actual account is actually unrighteous), and God declaring sinner to be righteous based on this imputation only, this imputation is received by faith alone. As you have already probably guessed the ideas which the Protestant quotes above condemned are the very ideas which the Catholic Church considers essential and orthodox. Most notably that justification is by infused righteousness, and not on the “righteousness of Christ” basis of Christ taking your punishment and perfectly fulfilling God’s commandments in your place. Here is how the Catholic Church defines justification in its essence, from the Council of Trent:
By which words, a description of the Justification of the impious [unrighteous] is indicated,-as being a translation, from that state wherein man is born a child of the first Adam, to the state of grace, and of the adoption of the sons of God, through the second Adam, Jesus Christ, our Saviour. And this translation, since the promulgation of the Gospel, cannot be effected, without the laver of regeneration, or the desire thereof, as it is written; unless a man be born again of water and the Holy Ghost, he cannot enter into the Kingdom of God.
CANON XI.-If any one saith, that men are justified, either by the sole imputation of the justice of Christ, or by the sole remission of sins, to the exclusion of the grace and the charity which is poured forth in their hearts by the Holy Ghost, and is inherent in them; or even that the grace, whereby we are justified, is only the favour of God; let him be anathema [excommunicated]. 
Justification for Catholicism is first and foremost about going from the state of “the first Adam” in which we lack sanctifying grace in our souls, to having that grace put back in our souls resulting in true inner righteousness and adoption as sons of God. This is in direct and irreconcilable contrast to the Protestant view of imputation. It is the difference between being truly righteous (“just”) in God’s eyes and merely appearing or accounted as righteous in God’s eyes though not being so inwardly.
Lets see how things fit together. Recall that the protestant view of Original Sin is not one in which sanctifying grace is lost and needs to be restored, but rather the corruption of the human nature in which man’s soul is like a “contaminated spring” which cannot produce pure water and is unsightly to God. With this in mind you can kind of understand why the Protestants believe in the imputation model, the “contaminated spring” must be overlooked, bypassed, covered, etc, or else God will never be able to see you as “righteous” and save you. The Catholic infusion model makes sense in that the undoing of Adam’s sin is restoring to the soul the sanctifying grace, in which the soul is “incomplete” without, and which God looks at to see who His adopted children are. Next fits in the views on the Atonement where for the Protestants using the imputation model need Christ to do for the corrupt man what he cant ever do, while the Catholic infusion model needs Christ to open the floodgates of grace to flow back into our souls to enable us to do what God requires of us.
Finally it is important to realize that when a Protestant uses the term “Justification by faith alone” it more accurately means "Justification by imputation of grace by faith alone,” where as Catholics teach it is by infusion of grace. Again this is NOT essentially a matter of “faith” for Protestants versus “works” for Catholics, but rather what justification means to each side.
What does the “blood” of Christ do?
If you look at other references to blood in the New Testament here are some verses you will find:
Heb 9: 12and not through the blood of goats and calves, but through His own blood, He entered the holy place once for all, having obtained eternal redemption. 13For if the blood of goats and bulls and the ashes of a heifer sprinkling those who have been defiled sanctify for the cleansing of the flesh, 14how much more will the blood of Christ, who through the eternal Spirit offered Himself without blemish to God, **cleanse your conscience **from dead works to serve the living God?
Heb 10: 29How much severer punishment do you think he will deserve who has trampled under foot the Son of God, and has regarded as unclean the blood of the covenant by which he was sanctified, and has insulted the Spirit of grace?
Heb 13: 12Therefore Jesus also, that He might sanctify the people through His own blood, suffered outside the gate.
1 Jn 1: 7but if we walk in the Light as He Himself is in the Light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin.
Rev 1: 5To Him who loves us and released us from our sins by **His blood **
Rev 7: 14I said to him, “My lord, you know.” And he said to me, "These are the ones who come out of the great tribulation, and they have washed their robes and made them white in the blood of the Lamb.
That is a real change not a cover up
Mat 23: 25"Woe to you, scribes and Pharisees, hypocrites! For you clean the outside of the cup and of the dish, but inside they are full of robbery and self-indulgence. 26"You blind Pharisee, first clean the inside of the cup and of the dish, so that the outside of it may become clean also. 27"Woe to you, scribes and Pharisees, hypocrites! For you are like whitewashed tombs which on the outside appear beautiful, but inside they are full of dead men’s bones and all uncleanness. 28"So you, too, outwardly appear righteous to men, but inwardly you are full of hypocrisy and lawlessness.
Luke 11: 39But the Lord said to him,** "Now you Pharisees clean the outside of the cup and of the platter; but inside of you, you are full of robbery and wickedness.** 40"You foolish ones, did not He who made the outside make the inside also? 41"But give that which is within as charity, and then all things are clean for you.
Look at the Matthew 23 and Luke 11 passages quoted above. Jesus explicitly condemns the idea of appearing righteous on the outside while leaving the inside filthy. Does it make sense for God to declare a unrighteous soul to be righteous because they “appear righteous” on the outside but in fact their soul is actually unrighteous? Catholics and the Scriptures clearly say such a teaching is unacceptable and even abominable.
This is what Christ’s death did for us
He gave us the gas you just have to put it in drive
hope this helps
by the way I stole this from Catholic Dude he want mind you can find the whole article here catholicdefense.googlepages.com/article.htm