Your question has nothing to do with god or religion. Forgiveness is simply the ability to give up a resentment toward someone who has done us a wrong, either perceived or real.
The only person who has any vested interest in forgiveness is the person who has been harmed either physically or emotionally.
Psychology teaches us that harboring ANY negative emotion to a point of obsession is harmful to the body and the mind. If one chooses to harbor ill feelings they simply allow someone else to intrude on their well being.
Forgiveness is a form of healing that is only attained by a person's willingness to disavow another person's power to affect their lives.
Non-forgiveness is simply the harmed person's self imposed punishment for something they had no control over. When carrying resentment becomes more burdensome than the positiveness of gaining sympathy from others for carrying it, it is time to discard it.
It may be more practical to continue the resentment, if it teaches us to avoid similar situations in the future, and we can go on with our lives with that knowledge instead of it being a crippling event.