From "Offer Forgiveness and Receive Peace" by Saint John Paul II (30th World Day of Peace, January 1, 1997):
Believers know that reconciliation comes from God, Who is always ready to forgive those who turn to Him and turn their back on their sins (cf. Isaiah 38:17). God's immense love goes far beyond human understanding, as Sacred Scripture says: "Can a woman forget her suckling child, that she should have no compassion on the son of her womb? Even these may forget, yet I will not forget you" (Isaiah 49:15).
Divine love is the foundation of the reconciliation to which all of us are called. "It is He Who forgives all your guilt, Who heals every one of your ills; Who redeems your life from the grave, Who crowns you with love and compassion.... He does not treat us according to our sins nor repay us according to our faults" (Psalm 103:3-4, 10).
In His loving readiness to forgive, God went even to the point of giving Himself to the world in the Person of His Son Who came to bring redemption to every individual and all humanity. In the face of human offenses, which culminated in His condemnation to death on the Cross, Jesus prayed: 'Father, forgive them; for they know not what they do" (Luke 23:34).
God's forgiveness is the expression of His loving kindness as our Father. In the Gospel parable of the prodigal son (cf. Luke 15:11-32), the father runs to meet his son as soon as he sees him coming home. He does not even let the son apologize: everything is forgiven (cf. Luke 15:20-22). The intense joy of forgiveness, offered and received, heals seemingly incurable wounds, restores relationships, and firmly roots them in God's inexhaustible love.
Throughout His life, Jesus proclaimed God's forgiveness, but He also taught the need for mutual forgiveness as the condition for obtaining it. In the Lord's Prayer, He makes us pray: "Forgive us our trespasses, as we forgive those who trespass against us" (Matthew 6:12). With that "as", He places in our hands the measure with which we shall be judged by God.
The parable of the unforgiving servant, punished for his hardness of heart towards his fellow servant (cf. Matthew 18:23-35), teaches us that those who are unwilling to forgive exclude themselves by this very fact from divine forgiveness: "So also My heavenly Father will do to every one of you, if you do not forgive your brother from your heart" (Matthew 18:35).
Our prayer itself cannot be pleasing to the Lord unless it is preceded, and in a certain sense "guaranteed" in its authenticity, by a sincere effort on our part to be reconciled with our brother who has "something against us": only then will it be possible for us to present an offering pleasing to God (cf. Matthew 5:23-24).
(picture source: https://presentationparish.org/faith-formation/what-divine-mercy)