Lack of forgiveness = disease and spiritual murder
It may be an infinitely less evil to murder a man than to refuse to forgive him. The former may be the act of a moment of passion: the latter is the heart's choice. It is spiritual murder, the worst, to hate, to brood over the feeling that excludes, that, in our microcosm, kills the image, the idea of the hated... In as far as we can, we quench the relations of life between us; we close up the passages of possible return. This is to shut out God, the Life, the One. For how are we to receive the forgiving presence while we shut out our brother from our portion of the universal forgiveness, the final restoration, thus refusing to let God be All in all? If God appeared to us, how could he say, "I forgive you," while we remained unforgiving to our neighbour? Suppose it possible that he should say so, his forgiveness would be no good to us while we were uncured of our unforgivingness. It would not touch us. It would not come near us. Nay, it would hurt us, for we should think ourselves safe and well, while the horror of disease was eating the heart out of us.
Tenfold the forgiveness lies in the words, "If ye forgive not men their trespasses, neither will your heavenly Father forgive your trespasses." Those words are kindness indeed. God holds the unforgiving man with his hand, but turns his face away from him. If, in his desire to see the face of his Father, he turns his own towards his brother, then the face of God turns round and seeks his, for then the man may look upon God and not die.
With our forgiveness to our neighbour, in flows the Consciousness of God's forgiveness to us; or even with the effort, we become capable of believing that God can forgive us. No man who will not forgive his neighbour, can believe that God is willing, yea, wanting to forgive him, can believe that the dove of God's peace is hovering over a chaotic heart, fain to alight, but finding no rest for the sole of its foot.
George MacDonald, "It Shall Not Be Forgiven," Unspoken Sermons, 32.