Another excerpt from R. T. Kendall's book. I think you would enjoy reading this book.

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“If you do not forgive men their sins, your Father will not forgive your sins,” Jesus said. Why do you suppose God so hates an unforgiving spirit? There are three reasons.

1. It shows an indifference to the greatest thing God did.

This “greatest thing” was God sending His Son to die on the cross for our sins. To be forgiven is the most wonderful thing in the world. But in order to forgive us, God paid a severe price. I predict that when we get to heaven we will be able to see, little by little, what it meant for God to send His Son to die on a cross. We now only see the tip of the iceberg. We see waves of glory, and these overcome us, but we’ve seen little. God did for us what we did not deserve. He therefore wants us to pass this on to others who don’t deserve it.

2. We interrupt God’s purpose in the world: reconciliation.

God loves reconciliation. He has given the ministry of reconciliation to us, and He wants it to continue. When we are forgiven, He wants us to pass it on. When we interrupt that, He doesn’t like it at all. He sent His Son to die on a cross, effectually calling us by His grace and giving us total forgiveness. But we interrupt that flow by not passing it on.

3. God hates ingratitude.

God knows the sins for which He has forgiven us, and He loves a grateful response. Matthew 18 relates the story of a servant who owed a great debt. He fell on his knees before his creditor, his master, and said, “Be patient and I will pay you back everything.” The master took pity on him, canceling the debt and letting him go. The master knew the things for which he had forgiven his servant. But then that same servant went out and found one of his own servants who owed him a much smaller amount; he grabbed the man and began to choke him, saying, “Pay back what you owe!” The fellow servant did exactly what he himself had done; he fell on his knees and said, “Please forgive me. I will pay you back.” But the one who had been forgiven a much greater debt refused to extend forgiveness, and he threw his servant into prison. To think there could be such ingratitude! Word eventually reached the original master, and the unforgiving servant was also thrown into debtor’s prison.

 R. T. Kendall, Total Forgiveness: When Everything in You Wants to Hold a Grudge, Point a Finger, and Remember the Pain - God Wants You to Lay It All aside (Lake Mary, FL: Charisma House, 2010).