Peter asked Jesus this question:

"Lord, how oft shall my brother sin against me, and I forgive him? till seven times?" - Matthew 18:21 KJV

Jesus answered:

"I say not unto thee, Until seven times: but, Until seventy times seven." - Matthew 18:22 KJV

"Therefore is the kingdom of heaven likened unto a certain king, which would take account of his servants. And when he had begun to reckon, one was brought unto him, which owed him ten thousand talents. But forasmuch as he had not to pay, his lord commanded him to be sold, and his wife, and children, and all that he had, and payment to be made.

“The servant therefore fell down, and worshipped him, saying, Lord, have patience with me, and I will pay thee all. Then the lord of that servant was moved with compassion, and loosed him, and forgave him the debt.

“But the same servant went out, and found one of his fellowservants, which owed him an hundred pence: and he laid hands on him, and took him by the throat, saying, pay me that thou owest.

“And his fellowservant fell down at his feet, and besought him, saying, Have patience with me, and I will pay thee all. And he would not: but went and cast him into prison, till he should pay the debt.

“So when his fellowservants saw what was done, they were very sorry, and came and told unto their lord all that was done.

“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." - Matthew 18:23-34 KJV

Then what did Jesus say?

"So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses." - Matthew 18:35 KJV

Even though the king had forgiven the servant his huge debt, when the servant did not forgive his fellowservant, the huge debt was put back on him, and he was delivered to the tormentors until it was all paid back.

This shows that we cannot take for granted to remain gone the huge dept of sin that the Lord has forgiven us of, if we will not forgive our brother the sin he has done against us.