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. [Matthew 6:14–15]
That our own forgiveness should be conditioned on forgiving others can be a hard doctrine, particularly if the sin against us is horribly wrong and out of all proportion to any harm we’ve ever committed. Even harder, the Lord has indicated in modern revelation that “he that forgiveth not his brother his trespasses standeth condemned before the Lord; for there remaineth in him the greater sin” (D&C 64:9). This is a very strong statement: if we refuse to forgive, there remaineth in us the greater sin. How can this be? As I hope to explain, our salvation is conditioned on forgiving others because when we refuse to forgive, what we are really saying is that we reject, or don’t quite trust, the Atonement. And it is our acceptance of the Atonement that ultimately saves us.
Faith to Forgive Grievous Harms: Accepting the Atonement,
James R. Rasband, BYU, October 23, 2012