"I, even I, am He that blotteth out thy transgressions for Mine own sake, and will not remember thy sins."

                            -Isaiah 43:25 

When the Bible talks about God forgetting our sins, it doesn't mean that the Almighty suddenly came down with a holy case of amnesia and has no knowledge that those things ever happened; it means that He has simply chosen not to remember those sins against us any more. In other words, our iniquities have been cast into God's sea of forgetfulness and the Lord will treat us as if they were never there. For some reason though, many people who have tasted of God's forgiveness still struggle when it comes to forgiving others.

"And forgive us our debts, as we forgive our debtors."

                            -Matthew 6:12

In order to be forgiven of our sins, we must forgive those who sin against us (Matthew 6:15). Jesus tells a story in Matthew 18 of a man who was forgiven of a debt equivalent to millions of dollars in today's currency, but yet this same man was unwilling to forgive a debt of 100 pence (about ten dollars). We, like the man in the story, have been forgiven of an enormous debt that we could not possibly pay. Why then is it so hard for us to forgive others?

"Then his lord, after that he had called him, said unto him, O thou wicked servant, I forgave thee all that debt, because thou desiredst me:
Shouldest not thou also have had compassion on thy fellowservant, even as I had pity on thee?
And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him.
So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses."

                            -Matthew 18:32-35

The Lord does not tolerate unforgiveness in the hearts of His people because it shows a lack of appreciation for the forgiveness that He has shown us.  And, as Jesus points out in Matthew 18:35, true forgiveness must come from the heart. Continuing to talk about someone for something that they did when you claim to have "forgiven" them for it is a sign that you haven't really forgiven them at all, for "...out of the abundance of the heart the mouth speaketh." (Matthew 12:34)

Let's put such things under the blood once and for all - you may be surprised how good it feels.