God commands us to love our neighbors and even our enemies. Love is greater than faith. Love is greater than hope. Love bears all things (1 Cor. 13:7). Unforgiveness is proof of a lack of genuine love in your heart; and the problem with the sin of unforgiveness is that it is an on-going condition of a person's heart.
One sad thing about harboring the sin of unforgiveness in your heart is---you're the loser. Concerning now, the apostle Paul wrote that we should forgive "lest Satan should get an advantage of us" (2 Cor. 2:11). And concerning eternity, it will involve, at least, the loss of rewards.
Peter said, "Lord, how oft shall my brother sin against me, and I forgive Him? till seven times? Jesus saith unto him, I say not unto thee, Until seven times: but, Until seventy times seven" (Mt. 18:21-22).
And Paul said, "Be ye kind one to another, tender-hearted, forgiving one another, even as God for Christ's sake hath forgiven you" (Eph. 4:32). "Forbearing one another, and forgiving one another, if any man have a quarrel against any: even as Christ forgave you, so also do ye" (Col. 3:13).
Even if you obey the Lord in all the New Testament commandments (about 180 of them), except the one to forgive everyone who has sinned against you, it is irrefutable proof that you do not love that person or those people, and you do NOT love Jesus Christ.
Jesus clearly said that if you love Him you will obey His (New Testament) commands, and if you don't obey (all) of them, you do NOT love Him.
Unforgiveness is a common sin today, even among true Christians. And it is a sin that is, in a sense, like the unpardonable sin of blasphemy of the Holy Spirit because of what Jesus said: "If ye forgive men their trespasses, your heavenly Father will also forgive you: but if ye forgive not men their trespasses, neither will your Father forgive your trespasses" (Mt. 6:14-15).