When I have offended another believer, I must seek reconciliation quickly, before they have had time to form a final judgment against me and form a grudge. I must not procrastinate. Once a grudge forms, I will be imprisoned outside of their lives, unable to give or receive the things God wants to give us through each other, unless and until God intervenes to change their heart toward me.

When I have offended another believer, I must seek reconciliation quickly and not procrastinate. How fast do I need to act? Before the brother or sister I have offended has had time to form a final judgment against me:

If I fail to act quickly enough, the offended brother or sister will judge me unworthy of fellowship–full partnership with them in the Body of Christ. This judgment will lead to them forming a grudge and they will begin to return evil for evil to hurt me. This taking of revenge can manifest as obviously as taking direct action to hurt me, as subtly as gossip, or even silently, by giving me the “cold shoulder,” ignoring me and denying me needed help, prayers or emotional support. The “silent treatment” is likely the most dangerous of these, because the offended person doing it will likely not recognize that they are holding a grudge. In their own mind, they will simply be protecting themselves from more hurt by ignoring me. But the grudge will nevertheless have its full destructive effect on both of us.
When the offended believer has formed a final judgment against me and begrudged me, I have been, in their life, cast into debtor’s prison. And Jesus said they will keep me there until I pay the last penny of the debt they believe I owe them for my sin. This is true whether the revenge taken is open, subtle, or only silent. I must pay all the revenge they believe they are owed before I will be released.
This formation of a final judgment of debt and unworthiness, and of a grudge, resulting in imprisonment, appears to be the same event Jesus later spoke of in his parable of the “unforgiving servant:”

Why were the other servants so sorry for the imprisoned debtor that they took it upon themselves to tell their Lord about it, even though the Lord was not a party to the debt?
They recognized that the result was very unfair. It was unfair in two ways. First, the servant who had imprisoned his fellow for a small debt had just been forgiven a much larger debt by his Lord. And second, they recognized that, by putting his fellow servant and debtor in prison, he had rendered him incapable of ever paying his debt. While in prison, the debtor could not work and so could never make any payments on his debt.
Unless their mutual Lord did something to relieve his distress, the imprisoned servant had no hope–he would remain in prison for the rest of his life.
I do the same thing to other believers when I form grudges against another believer and judge them unworthy of their place in my life. I confine them in a place from where, unless our mutual Lord intervenes, they will never be able to satisfy their debt to me. They are permanently imprisoned outside of my life, and so prevented from giving any future help or encouragement God would have wanted me to give them. I also am permanently prevented from receiving from them any help or encouragement God would have given me through them in the future.
Further, because we are both involved in a complex web of relationships with many of the same people in the Church, my debtor’s imprisonment in my life will diminish the availability of both of us to provide what God wants us to provide to other people.
So the debtor’s “prison” Jesus is talking about describes a very real part of my experience–and that of my debtor–when I form a final judgment and start to hold a grudge.
It is serious business, and Jesus tells that if I am the offender I must seek reconciliation quickly, before it has a chance to happen.
Otherwise, I will end up in debtor’s prison, and it will take divine intervention to get me out, if I ever get out at all. God will have to change their heart toward me, causing them to recognize that I am still their brother and that Jesus has already paid the debt of all of my sin, even my sin against them. But he will not force their will–he will not make them release me. And the things he will allow to happen to move them toward forgiveness are likely to be quite unpleasant for both of us. So it is far better if I seek their forgiveness promptly, before the grudge sets in!