Each and every person ever born onto this earth will also, one day, exit this earth.
The Lord Jesus clarified many times to us that a state of unforgiveness will bar us from entering Heaven. Thus, there will be no excuse for any of us not to have known.
Unforgiveness may be thought to include the emotional states of "holding a grudge" or having a noted dislike for someone.
It can also be extended to include negative feelings, irritation or anger towards another. These types of emotions are all displays of unforgiveness towards our "neighbor".
The Parable of the Unforgiving Servant in the Book of Matthew conveys our relationship with both God and with others. It portrays how Jesus paid our debt that He didn't owe and that we ourselves, could never pay.
This unmerited pardon of the insurmountable debt of our sins by the Father is illustrated by the "certain king" who forgave the debt of the servant who owed much.
So are our actions when as Believers, who desire God's compete forgiveness, choose instead, not to forgive others.
"Therefore, the kingdom of heaven is like a king who wanted to settle accounts with his servants. As he began the settlement, a man who owed him ten thousand talents was brought to him. Since he was not able to pay, the master ordered that he and his wife and his children and all that he had be sold to repay the debt.
"The servant fell on his knees before him. 'Be patient with me,' he begged, 'and I will pay back everything.' The servant's master took pity on him, canceled the debt and let him go.
When the other servants saw what had happened, they were greatly distressed and went and told their master everything that had happened.
In anger his master turned him over to the jailers to be tortured, until he should pay back all he owed.
Preceding the above Parable, note that Peter had asked of Jesus:
"Lord, how many times shall I forgive my brother when he sins against me? Up to seven times?" Matthew 18:20
Note Jesus's response:
"Jesus said to him, “I do not say to you seven times, but seventy-seven times". Matthew 12:21
As such, even when we encounter a repeat offender we are to repeatedly forgive, with sincerity from our heart.
"And forgive us our debts, As we forgive our debtors. " Matthew 6:12
Holding on to feelings of unforgiveness never harms the source/perpetrator. It only harms the one harboring it.
In fact, the object of the unforgiving attitude is often virtually unaware and totally unaffected. Meanwhile. the one holding the unforgiveness in their heart is not only emotionally and spiritually miserable, but actually in the danger of the fires of Hell.
In conclusion, harboring unforgiveness only hurts us during this earthly life by removing our peace and joy.
As such, it can actually cause an emotional "hell on earth" as well as the terrifyingly real potential of literally sending one to Hell for eternity.
Jesus made it perfectly clear that our sins are not forgiven us if we do not forgive others. If our sins are not forgiven, we cannot enter into the presence of God.
The holy and righteous God cannot tolerate sin in His presence. Therefore, if we choose to retain unforgiveness, then we also retain our sins.
"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