I found this gem from St. Gregory Palamas: The Homilies, trans. by Christopher Veniamin, Mt. Thabor Publishing, 2009, p. 297.
The Lord pitied us when we repented, and forgave us much, but when He sees how heartless and unyielding we are to our fellows, His wrath is justly kindled against us, and He delivers us up to unendurable torments, both temptations here, and unending future punishments, in accordance with the words of the Truth Himself. "So likewise, says the Lord, "shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses" (Matt. 18:35). What is even more terrible is that we shall find, alas, that God's true servants, the holy angels and the saints, whom we hope by our prayers here to have as ambassadors and intercessors to God on our behalf, will be inciting Him against us, if in this present life we fail to show compassion to those who offend against us. This is because the holy angels and saints are those whom, as we learned earlier, with great sorrow denounce to the Lord our hard and merciles frame of mind.
Lack of forgiveness not only hurts our soul and can lead to eternal damnation, but also causes physical and mental illness, so that we shall suffer torments in this life and in the next.
In addition, St. Gregory mentions, "each of us has sinned more frequently than that against God. If you look carefuly at how great one sin is, bearing in mind against Whom it is committed, you will find it incomparably worse than all your brother's sins, be they seventy times seven" (ibid, p. 297).
Furthermore, St. Gregory writes that those who offend us offer us the opportunity to pay back our debt to God. In this, we learn patience and love. Finally, St. Gregory cautions us to beware of our adversary the devil. We can defeat his diabolical plans by quickly offering compassion to our fellow man who has offended us (ibid. p. 298).
