For Christians, to forgive others is required to spend eternity with Jesus Christ.
When we obey the Gospel of Jesus Christ (when we are born again) all of our sins are forgiven, are remitted:
Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost (Acts 2:38).
In the middle of the Lord's Prayer (Matthew 6:9-13), Jesus says:
And forgive us our debts, as we forgive our debtors (Matthew 6:12).
And immediately after Jesus concludes the Prayer, He says:
For 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 (Matthew 6:14-15).
So even though our sins were forgiven when we were born again, He made it very clear that no matter how long we have been saved, He still requires us to forgive others of their trespasses (debt, sins) if He is to forgive us of our trespasses. Having said that though, the "but" in the scripture means that forgiveness of others is a choice. The problem is that it is a heaven or hell choice! Yes, once saved does not mean you are always saved. Go here to view that lesson.
Let's look at other scriptures where Jesus expounded on the principle that Christians will not make it to heaven if they do not forgive others. In the parable of the unforgiving spirit (Matthew 18:21-34), Peter asks how many times we should forgive others (Matthew 18:21), Jesus responded:
I say not unto thee, Until seven times: but, Until seventy times seven (Matthew 18:22).
If we have an unforgiving spirit, how could we possibly forgive someone four hundred and ninety times times? Jesus then went on to teach the disciples in the parable that to not forgive others will result in Him applying our original debt (trespass, sin) back on us. In this parable, the servant's lord had forgiven the servant of his debt (Matthew 18:27). Then when the servant's lord discovered that this servant had not forgiven a debt that was owed to him (Matthew 18:32-33), the lord delivered the servant "to the tormentors" (Matthew 18:34). Jesus then says:
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).
Like Jesus said to forgive a person must be from our hearts, meaning just not out from our mouths, otherwise we will be thrown to the torment of hell. Is there anybody we have not forgiven? If so let's make a point to repent and to forgive that person. If we still can't we need to go to the throne room and ask Jesus to help us to change our heart, giving us a forgiving spirit.