vatican.va/archive/ENG0015/__PAB.HTM
2841 This petition is so important that it is the only one to which the Lord returns and which he develops explicitly in the Sermon on the Mount.137 This crucial requirement of the covenant mystery is impossible for man. But "with God all things are possible."138
. . . as we forgive those who trespass against us
2842 This “as” is not unique in Jesus’ teaching: “You, therefore, must be perfect, as your heavenly Father is perfect”; “Be merciful, even as your Father is merciful”; "A new commandment I give to you, that you love one another, even as I have loved you, that you also love one another."139 It is impossible to keep the Lord’s commandment by imitating the divine model from outside; there has to be a vital participation, coming from the depths of the heart, in the holiness and the mercy and the love of our God. Only the Spirit by whom we live can make “ours” the same mind that was in Christ Jesus.140 Then the unity of forgiveness becomes possible and we find ourselves “forgiving one another, as God in Christ forgave” us.141
2843 Thus the Lord’s words on forgiveness, the love that loves to the end,142 become a living reality. the parable of the merciless servant, which crowns the Lord’s teaching on ecclesial communion, ends with these words: "So also my heavenly Father will do to every one of you, if you do not forgive your brother from your heart."143 It is there, in fact, “in the depths of the heart,” that everything is bound and loosed. It is not in our power not to feel or to forget an offense; but the heart that offers itself to the Holy Spirit turns injury into compassion and purifies the memory in transforming the hurt into intercession.
vatican.va/holy_father/john_paul_ii/encyclicals/documents/hf_jp-ii_enc_30111980_dives-in-misericordia_en.html
Christ emphasizes so insistently the need to forgive others that when Peter asked Him how many times he should forgive his neighbor He answered with the symbolic number of "seventy times seven,"131 meaning that he must be able to forgive everyone every time. It is obvious that such a generous requirement of forgiveness does not cancel out the objective requirements of justice. Properly understood, justice constitutes, so to speak, the goal of forgiveness. In no passage of the Gospel message does forgiveness, or mercy as its source, mean indulgence towards evil, towards scandals, towards injury or insult. In any case, reparation for evil and scandal, compensation for injury, and satisfaction for insult are conditions for forgiveness.