Is the Roman Catholic understanding of forgiveness different from the Protestant understanding? Recently I read Simon Wiesenthal's book, The Sunflower: On the Possibilities and Limits of Forgiveness, and I was challenged to flesh out my understanding of forgiveness. There were aspects which I liked of the understandings of forgiveness by Wiesenthal, other characters in the book, and respondents to the question of whether he could or should have forgiven the Nazi who appeared to be repentant of his crimes against the Jews, but I wasn't entirely satisfied with the approaches offered. 

Though there were basically three sides offered to the answer (the Jewish, Roman Catholic, and Protestant traditions), the responses were all quite short and I didn't think that there was much in the way of textual support--either from the Bible or the written traditions of the Jews or the Roman Catholic church. What I'm looking for is an explanation of the Roman Catholic or Protestant (and/or Jewish too) understanding of forgiveness, with references. I'm not interested in the subjective, feelings-based approach which many of the essay responses gave (in the second half of the text).

Edit: The Jewish response is usually something like: "You have no right to forgive someone for sins they have done against someone other than yourself, even if you are part of a group whom the apparently repentant sinner has sinned against (i.e. one Jew has no right to forgive someone for sins against other Jews but only for sins against himself). I think I agree with this. The Roman Catholic response (in the book) seems to be fairly similar to the Jewish one, but the generic Protestant response in this book seems to be that people should forgive anyone and everyone regardless of repentance(seemingly more for the sake of the person doing the forgiving than for the sake of the person being forgiven). I'm not trying to pin a position on Roman Catholics or Protestants--that's just the way it seemed to me from reading the book. So I'm trying to understand whether the book gave a representative view of these positions and I'm also trying to understand for myself who can be forgiven, by whom, and under what circumstances? To add more to the question (than the book did), in Roman Catholic or Protestant teaching, is it impossible to forgive someone whom God has not forgiven? Please do indicate what tradition you are responding from.
A Catholic Understanding
There are two parts to your question:

Can one forgive another for a wrong they have not suffered personally?
Can one forgive another if God has not forgiven them?

One at a time.
Can one forgive another for a wrong they have not suffered personally?
If one hasn't suffered a wrong personally, it's not clear that there's anything to forgive, or be forgiven, in a technical sense. I might feel outraged as a Catholic by the way Catholics have historically been treated, or as an Irish citizen (at least technically) by the way the Irish have often been treated; but that doesn't mean there's anything for me to forgive. I am required to live in charity with my neighbor, however; and to that extent I may be said to &quot;forgive&quot; someone if I put aside my anger at them (an anger I have developed on behalf of another) and dwell with them in peace. Indeed I must do so:

When you stand to pray, forgive anyone against whom you have a grievance, so that your heavenly Father may in turn forgive you your transgressions.

(Mark 11:24)
Can one forgive another if God has not forgiven them?
From a Catholic perspective, there are a few things to be addressed here. First of all: Should one forgive a person who does not appear to have sincerely expressed repentance? Second: Assuming that someone has expressed what appears to be sincere repentance, is it possible that God does not forgive them? Finally: is it possible, or morally right, to forgive someone whom God does not forgive? Let's take those individually.

Should one forgive another who has not sincerely repented?

In his discussions of forgiveness, Jesus nowhere appears to put conditions.

Then Peter approaching asked him, &quot;Lord, if my brother sins against me, how often must I forgive him? As many as seven times?&quot; Jesus answered, &quot;I say to you, not seven times but seventy-seven times.&quot;

(Matthew 18:21–22; New American Bible, Revised Edition)
The NABRE note to this text points out the similarity to Genesis 4:24, and notes:

In any case, what is demanded of the disciples is limitless forgiveness.

Further, the Catechism of the Catholic Church (in the context of its discussion of family life) states:

Each and everyone should be generous and tireless in forgiving one another for offenses, quarrels, injustices, and neglect. Mutual affection suggests this. The charity of Christ demands it.

(paragraph 2227)
Thus, it appears that from a Catholic perspective, yes; one should and indeed must forgive another regardless of whether they have repented or not.

Assuming that someone has sincerely repented, is it possible that God does not forgive them?

No.

&quot;There is no one, however wicked and guilty, who may not confidently hope for forgiveness, provided his repentance is honest.&quot;

(Catechism of the Catholic Church, paragraph 982, quoting the Roman Catechism, Part I, chapter 11, paragraph 5)

Is it possible, or morally right, to forgive someone whom God does not forgive?

We can never know for certain whom God has forgiven, and whom he has not. God is not bound by his Sacraments, and has many ways of forgiving, even up to the point of death. Further, we have the two prior statements: that we must forgive others regardless of circumstances, and that God will forgive one who sincerely repents.