God and Goddess's work and glory is to bring about the immortality and eternal life of humankind: to raise us up as joint heirs of Christ: that we become like Him, and one with Them.
That unity is only possible with perfect love, thus the two great commandments: love God, love each other. To break these commandments is to cause separation via withdrawal or alienation.
Repentance is the process of transformation that makes us more like Christ, that corrects what we are doing wrong, and that brings us closer to unity.
Forgiveness is the recognition of (potential) repentance: a refusal to forgive is a refusal to be one, a denial of another's repentance or even the possibility of their repentance. We cannot be one if we deny the other a place at the table. If we are not one, we are not Christ's people.
At the end, failing to forgive is a denial of the power of Christ unto the transforming of the human soul, it is standing at the door of His house telling others that they may not enter, rejecting them and saying that they may not be one with our Parents or our Savior.
God knows the intents of our hearts and is able to judge our repentance, our willingness to change. Some will fall short and be unable to forgive themselves (to stand in Their presence), or others (losing unity and thus falling out of Their presence). We cannot know or judge the hearts of others and so are commanded to forgive ALL.
When forgiveness is applied to the question of justice versus mercy, several things become clear. First, in order for justice to be needed, injury has to have been given. All of the injuries that we commit upon each other require justice, *unless* we forgive each other, relinquishing our claim to justice and showing mercy to our fellow beings. Thus mercy can be given without robbing justice and we may all sit down in our Parents' kingdom.
A coda: I still have much to learn of forgiveness, and expect that what I have written here will seem woefully incomplete in twenty years. For example, just today I was struck with a sense of 'looking upon the heart' - that perhaps I need to be more generous and forgiving of others' attempts to do good, since my own attempts are just as clumsy and awkward.