sin covering

''Sin-covering is one way of describing what takes place at justification, because the guilt and eternal punishment due to all our past sins are removed. Man is a temporal being, and we do not escape from our past merely by its being past. Our past eternally remains our past, our history. Justification does not make those past sins never to have happened. They are covered, as it were, through our union with Christ in His satisfactory sacrifice, by which something more pleasing to God than all our sins are displeasing is offered to the Father. But justification-as-translation does effect an immediate *present* transformation.''

found in the comment in this link  http://www.calledtocommunion.com/2013/01/holy-church-finding-jesus-as-a-reverted-catholic-a-testimonial-response-to-chris-castaldo/#comment-47048

see the cartoon picture on the above link and here is more of a quote:

In the Reformed view, that cartoon is true of every believer for every moment of his life as a believer, even in his moments of greatest sanctification. Inside he is a filthy rotten sinner his whole life long, but God covers his sin with the righteousness of Christ imputed to him.
In Catholic doctrine, by contrast, God does not cover sin in that sense. Rather, God covers sin by removing it at the moment of justification, so that it belongs only to the past.

from comment 20  http://www.calledtocommunion.com/2011/11/the-doctrine-of-merit-feingold-calvin-and-the-church-fathers/#comment-187580

Regarding St. Thomas on the stain of sin, I’ve explained that in “Aquinas and Trent: Part 4.” As I show there, St. Thomas very clearly shows that the stain of sin is removed by grace. The stain of sin is not the sinful act. And the covering is the removal of the guilt and debt, without removing the past act (i.e. making the past to never have happened), as I’ve explained in comment #121 of the “Justification: The Catholic Church and the Judaizers in St. Paul’s Letter to the Galatians” thread, and in comment #69 under the “Holy Church” article. As for “covering,” St. Thomas does explain this in his commentary on St. Paul’s epistle to the Romans, in which St. Thomas writes, “For sins are said to be covered from the divine gaze, inasmuch as he does not look upon them to be punished.” (336) Covered sins are not looked upon to be punished, because their guilt and debt have been removed.

As for the Great Catechism, every statement you cited from it is fully compatible with the truth of what I have already said. The everlasting and unceasing character of Christ’s mediation does not mean or entail that guilt remains in the saints. It means rather that the guilt is permanently vanquished.