Jesus teaches a vital lesson on forgiveness: only those who repent can be truly be forgiven. And yet we must be ready to forgive everyone just as God is, even those guilty of the vilest of sins. And we should pray for that repentance and desire it just as He does. So if God can forgive sinners then we must also. And we have to forgive over and over again as long as it is necessary.
The disciples knew what Jesus was talking about. They knew how difficult it was to forgive as God does. This is impossible for man since we are so sensitive and demand justice and, at times, revenge. This is why the disciples asked for more faith, more power, to do this. But Jesus told them that faith involves trusting Him to accomplish the impossible. They were to submit the matter of forgiveness to God, trusting Him to do through them what they could not.
Once they did forgive, however, they were not to think that they have done anything special, or that they should be proud of themselves. They have only done their duty. Forgiveness is the duty of every Christian. God has canceled an enormous debt of sin for each of us. We have no right to withhold forgiveness of a debt which is minuscule in comparison. So as we consider the words of Jesus, we must remember how much we have been forgiven. Then we can approach and challenge those who offend us to repent and do so with humility and compassion.