Q&A: What if a believer chooses NOT to forgive someone?
There are really two parts to your question:
1. What is Jesus saying in Mark 11: 25-26 about forgiving others? And,
2. What if a believer chooses not to forgive someone?
First of all, the context of the passage in question from Mark 11 is PRAYER and the things that hinder it for us. Jesus cited unforgiveness as something that greatly hinders prayer because it breaks fellowship with God.
I understand that a lot of people read Mark 11:25-26 and immediately interpret it as saying that forgiving others seems to be listed here as a REQUIREMENT for salvation. They hear Jesus saying, "If you don't forgive others, God won't forgive you" and they assume that since a person isn't forgiven they are also not saved.
But that is NOT what Jesus is saying in that passage. The forgiveness that results in eternal life is, and always will be, a matter of GRACE through FAITH in the finished work of Jesus on the cross. In other words, it's bestowed as a FREE GIFT which Paul says is NOT OF WORKS. (Ephesians 2:8-9)
That means what Jesus is talking about in Mark 11 is something unrelated to salvation. He is speaking of God's parental dealings with His children and making us aware that an unforgiving spirit in a child of God causes a break in fellowship with the Lord that creates a hindrance for prayer.
What you were taught is true: when we come to Christ ALL our sins are forgiven — past, present and future. But that's referring to the forgiveness that results in salvation. There are other issues related to forgiveness that do NOT affect our salvation, but they DO affect our relationship with God. In other words, once a child of God is SAVED and trusting Jesus for forgiveness, their sin can no longer threaten their eternal salvation. But it CAN and DOES threaten their closeness to God. When you and I sin it STILL separates us from God relationally. It drives a wedge between us and we find it hard to pray and fellowship with God until we come to Him and confess our wrongdoing. Once we do, the relationship is mended and we can carry on in peace.
The sin that once threatened your eternal destiny has been dealt with at the cross. That is a done deal thanks to the death and resurrection of Jesus. But the daily mess ups and mistakes that we make must still be brought to the Lord in repentance so that nothing stands in the way of our daily walk with Christ. That's why we were taught to pray, "Forgive us our debts, as we have forgiven our debtors." (Matthew 6:12) Jesus wasn't telling us to get saved all over again. He was making sure nothing gets in the way of our closeness to God.
So what if a believer chooses NOT to forgive? Well, for starters they will be miserable. You have to know that God's Holy Spirit will be working overtime to get their attention so that they will humble themselves and repent of their hard heart. God is fully able to communicate that unforgiveness is an attitude entirely contrary to His will, and I trust that He will bring all the conviction needed to eventually cause that person to repent. I believe that God will eventually have His way — even if He has to bring some kind of suffering to do it. One way or another, they will humble themselves before their God.