Jesus tells Peter to forgive his brother “seventy times seven times” (Matthew 18:22). To explain what He means, Jesus tells the story of the servant whose huge debt was forgiven by the king after the servant begged him. Immediately afterwards the same servant found a fellow servant who owed him a much smaller amount. The servant put his fellow servant in jail. When the king discovered what he had done, he asked him: “Should you not have dealt mercifully with your fellow servant, as I dealt with you”? (Mt 18:33).
Clearly, the point is that we should be merciful to others as God is merciful with us.
St. Thomas Aquinas reflects: “We are induced to be merciful by the example of Divine mercy… ‘Be merciful, as your Father also is merciful’ (Lk 6:36)” (3a, 84. 10).
Thomas asserts that forgiveness is always offered to us through the Sacrament of Penance: “Our Lord commanded His disciples to be merciful by frequently pardoning their brothers who sinned against them … ‘seventy times seven.’ God, over and over again, through the Sacrament of Penance, grants pardon to sinners, especially as He teaches us to pray: ‘Forgive us our trespasses, as we forgive those who trespass against us’ (Mt 6:12) (3a, 84. 10).
Thomas recalls a group in the early church, who believed that once charity was possessed through Penance, the person could not lose charity and so they did not need Penance again. Thomas reflects that these people did not realize that charity can be lost by our deliberate choices. He concludes that they did not appreciate “the gravity of sin.”
Others in the early church claimed that those who sin again after being forgiven cannot be forgiven a second time. Thomas considers this to be a more serious error because it was “against the infinity of Divine mercy, which surpasses any number and magnitude of sins” (3a, 84. 10).
Thomas concludes: “God’s mercy, through the Sacrament of Penance, grants pardon to sinners without any end… It is therefore evident that Penance can be repeated many times” (3a, 84. 10).
Thomas explains that the power of the Sacrament of Penance is an effect from Christ’s Passion: “Penance has its power from Christ’s Passion as a spiritual medicine which can be repeated frequently” (3a, 84. 10).
According to St. Thomas: “Mercy is accounted as being proper to God: and therefore His omnipotence is chiefly manifested” (2a2ae. 30, 4).
Thomas says: “Of all the virtues which relate to our neighbor, mercy is the greatest, even as its act surpasses all others” (2a2ae. 30, 4).
 When a reference to St. Thomas’ Summa Theologiae is made, the part of the Summa is given first, in this case, the third part, followed by the question, in this case, question 84, followed by the article, in this case, article 10.