Forgiving others is an absolute necessity
"For if you forgive men when they sin against you, your heavenly Father will also forgive you. But if you do not forgive men their sins, your Father will not forgive your sins" (Matthew 6:14-15).
Jesus says we must forgive often
Then Peter came to Jesus and asked, "Lord, how many times shall I forgive my brother when he sins against me? Up to seven times?" Jesus answered, "I tell you, not seven times, but seventy-seven times" (Matthew 18:21-22).
So watch yourselves. "If your brother sins, rebuke him, and if he repents, forgive him. If he sins against you seven times in a day, and seven times comes back to you and says, ‘I repent,’ forgive him" (Luke 17:3-4).
Jesus commands us to forgive others
"And when you stand praying, if you hold anything against anyone, forgive him, so that your Father in heaven may forgive you your sins" (Mark 11:25).
Joseph forgave his brothers for treating him unkindly
"But Joseph said to them, "Don’t be afraid. Am I in the place of God? You intended to harm me, but God intended it for good to accomplish what is now being done, the saving of many lives. So then, don’t be afraid. I will provide for you and your children." And he reassured them and spoke kindly to them" (Genesis 50:19-21).
Put away bitterness and anger; forgive as God forgives
"Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice. Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you" (Ephesians 4:31-32).
It is sinful not to forgive
Forgive a repentant sinner and affirm you love
"Now instead, you ought to forgive and comfort him, so that he will not be overwhelmed by excessive sorrow. I urge you, therefore, to reaffirm your love for him" (2 Corinthians 2:7-8).