Today, we can see in the Gospel, how Peter asks Jesus about a very concrete theme that is still to be found in the heart of many persons: he asks the question about the limit of forgiveness. The reply is that this limit simply does not exist: «No, not seven times, but seventy-seven times» (Mt 18:22). And to explain this reality, Jesus uses a parable. The king's question centers the theme of the parable: «Weren't you bound to have pity on your companion as I had pity on you?» (Mt 18:33).
Forgiveness is a gift, a grace flowing out of God's love and mercy. For Jesus' forgiveness has no limits, provided repentance is true and sincere. But it requires opening our heart to conversion, that is, do with others as God requests us to.
Grave sin is excluded from the Kingdom (cf. Catechism of the Catholic Church n. 1470). In converting to Christ through the Sacrament of Penance and faith, the sinner passes from death to life, and the penitent's act crowning this conversion is his atonement. Our own deeds showing our expiation are the sign of our personal commitment —that the Christian has assumed before God— to begin a new existence, while repairing, wherever possible, whatever damage made to our neighbors.
There cannot be any forgiveness of sins without a minimal satisfaction, the finality of which is: 1. To avoid sliding over towards graver sins; 2. To reject sin (as expiation acts like a brake and makes the penitent more prudent and cautious); 3. To forsake, through virtuous deeds, the bad habits acquired with our bad life; 4. To resemble to Christ.
As St. Thomas Aquinas explains, «Man becomes God's debtor in two ways; first, by reason of favors received, secondly, by reason of sin committed: and just as thanksgiving or worship or the like regard the debt for favors received, so satisfaction regards the debt for sin committed». The man of the parable was not willing to behave according to the favor received, so he was no longer deserving forgiveness.