Forgiveness is a mandatory Christian virtue.
Jesus makes this very clear in several places in the Gospels, for example in the Sermon on the Mount.
For if you forgive other people when they sin against you, your heavenly Father will also forgive you. But if you do not forgive others their sins, your Father will not forgive your sins.
But Jesus makes it most clear of all in the Parable of the Unforgiving Servant.
The kingdom of heaven is like a king who wanted to settle accounts with his servants. And having begun to settle accounts, one debtor was brought before him who owed ten thousand talents. But because he had nothing to pay the debt, his lord commanded him to be sold, and his wife and children, and all that he had, and payment to be made.
The servant fell down, and bowed before him, saying, “Lord, have patience with me, and I will pay you everything.” The lord of that servant, being moved with compassion, released him, and forgave him the debt.
But that servant went out, and found one of his fellow servants who owed him one hundred denarii, and he grabbed him, and took him by the throat, saying, “Pay me what you owe!”
His fellow servant fell down at his feet and begged him, saying, “Have patience with me, and I will pay you everything.”
But he would not, and went and cast him into prison, until he would pay the debt. When his fellow servants saw what had been done, they were greatly distressed, and went and told their lord all that had happened.
Then his lord called him, and said to him, “Wicked servant! I forgave you all that debt, because you pleaded with me. Shouldn’t you have dealt kindly with your fellow servant, as I dealt kindly with you?” And in anger his lord delivered him to the tormentors, until he would pay all that was owing to him.
“So also my heavenly Father will do to you, if each of you does not forgive your brother from your heart.”
When we say the Lord’s Prayer, we pray that our heavenly Father will
Forgive us our debts, as we also forgive our debtors.
In other words, we pray that God will deliver us to the tormentors, if we do not forgive others from the heart! It helps to remember this.