Forgiveness is close to the core of Christianity, to forgive is mandatory - the implication is that we will be forgiven in the same manner and to the same extent that we ourselves forgive - yet I have found it very difficult to understand the concept.
What happens when we forgive?
In normal terms forgiveness seems much like forgetting, 'not thinking about' something - but that can't be correct in a heavenly perspective, because everything is 'remembered'.
Nor is forgiveness a matter of ignoring sin, or of making oneself believe that there is no difference between good and evil, or that what seemed bad was actually good (as when people always put the most optimistic and well-intentioned construction on events, regardless of the reality).
But I feel closer to understanding forgiveness having read the first four chapters of Charles Williams He Came Down From Heaven.
I have tried to read this book many times, but this time it 'clicked' and the book strikes me as without doubt one of the most profound theological writings I have encountered - almost alongside Pascal.
In essence, and as far as I understand it, to forgive is to put events into the ultimate and heavenly perspective when even the most deliberate evil is is seen as unable wholly to escape from Good, and will become an occasion for good.
The necessity to forgive is then, perhaps, an order not to despair; an injunction to be aware that God created all things, and makes them happen; that evil can destroy but Good is primary.
At root, the injunction to forgive is a statement of the nature of reality.
Note added: Conversely, failure to forgive - i.e. persistence of resentment, or grudge - is implicitly acceptance of the primacy of evil, the dominance of the demonic. It is denial of Christian Love as the ultimate principle. It is therefore the denial of reality.