You are right in that the majority of case studies, reported in the media, of people forgiving offenders for extreme cases of injustice seem to possess a deep faith. If you look at the News items on this website, you will see many such cases. Not all are Christian as seen, for example, in Eva Moses Kor’s forgiveness of Nazis who imprisoned her and her twin sister at Auschwitz. Mrs. Kor is Jewish.
Aristotle taught over 2,000 years ago that there are developmental movements in forgiveness from superficial to deep and profound. Most people can forgive others for small issues and we have worked with people from various belief systems (and no belief at all) to forgive significant injustices. Yet, the extreme injustices, again as reported in the media, do point to the theme of transcendence. By “transcendence” I mean going beyond the material, the concrete, what can be sensed in this world, to something more—something bigger. I think this theme of transcendence is important and worth taking seriously with regard to your question. Those who see that there is more to the body, more to this life seem to have the capacity to transcend resentment in a way that, as you suggest, is surprising.
I am not implying that atheists or agnostics cannot or will not transcend in their forgiving. I am saying that it may—it may—be harder for them to do so because materialistic philosophies do not assume that there is more beyond the physical bases of existence.