What does Matthew 18:31 mean?

Outrage would be an appropriate emotional response to what the servant in Jesus' story has just done. The man was forgiven by the king for an astounding amount of debt instead of being sold into slavery with his wife and children (Matthew 18:23–27). However, that same servant immediately demanded payment for a relatively tiny debt owed to him from a fellow servant. When that servant could not pay, the recently-forgiven man had his co-worker thrown into debtor's prison.

The other servants of the king are described by Jesus as deeply disturbed by this. They knew both sides of the story. They knew that, incredibly, the accusing man had been forgiven for an impossible sum. They knew that, unbelievably, the man had refused to forgive or even negotiate the much smaller sum. The ugliness of that action got to them. It could not stand. They went and reported to the king what had happened.

The point of Jesus' parable is becoming clearer. God is like the king in that He remains patient even as we accrue a massive "debt" of sin. In Christ, He forgives our enormous, unpayable debt of sin, the sin that has earned us His wrath and judgment. After Jesus' death and resurrection, this story would resonate even more deeply with the disciples. After all, Jesus is the one who paid the debt for our sin with His life. We are forgiven because He suffered and died (John 3:16–18; Romans 5:8).

Soon, Jesus will tie this parable to Peter's question about forgiveness (Matthew 18:21–22). The point is that it is outrageous for those who have been forgiven such an extraordinary burden of sin debt—at the cost of the life of Jesus—should refuse to forgive a tiny-by-comparison amount of sin inflicted on us by another.