"For, if ye forgive men their trespasses your heavenly Father will also forgive you; But if ye forgive not men their trespasses neither will your Father forgive your trespasses."
This is an absolute condition. It is mandatory.
If you forgive not men their trespasses, neither will your Father forgive you your trespass.
You can't be forgiven by the Father if you do not forgive others.
It can't be done.
That grudge you harbor prevents the Father from forgiving you.
Those resentments you think are justified are keeping you from being forgiven by the Father.
Those injustices imposed upon you by others who are unthinking or cruel must be surrendered.
The early Saints were victimized by mobs in Missouri and Illinois. They wanted revenge. Brigham Young implemented a covenant to seek vengeance upon the murderers of Joseph Smith until the third and fourth generation. They did not build Zion.
The opposite of this is forgiveness. If you forgive, your Heavenly Father WILL forgive you. Offenses are opportunities for you to gain forgiveness. All you need to do is forgive them.
It is a simple, direct cause and effect. It was ordained before the world was founded, and applies universally in all ages and among all people.
The world is in Satan's grip largely because the world seeks vengeance and refuses to forgive.
Zion, on the other hand, will be filled with those who forgive. Of course that puts an absolute limit on those who can dwell there. ...Very few indeed.