23 He has clearly joined herewith and added the law, and has bound us by a certain condition and engagement, that we should ask that our debts be forgiven us in such a manner as we ourselves forgive our debtors, knowing that that which we seek for our sins cannot be obtained unless we ourselves have acted in a similar way in respect of our debtors. Therefore also He says in another place, “With what measure you mete, it shall be measured to you again.” And the servant who, after having had all his debt forgiven him by his master, would not forgive his fellow-servant, is cast back into prison; because he would not forgive his fellow-servant, he lost the indulgence that had been shown to himself by his lord.
And these things Christ still more urgently sets forth in His precepts with yet greater power of His rebuke. “When you stand praying,” says He, “forgive if you have anything against any, that your Father which is in heaven may forgive you your trespasses. But if you do not forgive, neither will your Father which is in heaven forgive you your trespasses.” There remains no ground of excuse in the day of judgment, when you will be judged according to your own sentence; and whatever you have done, that you also will suffer.
For God commands us to be peacemakers, and in agreement, and of one mind in His house; and such as He makes us by a second birth, such He wishes us when new-born to continue, that we who have begun to be sons of God may abide in God's peace, and that, having one spirit, we should also have one heart and one mind. Thus God does not receive the sacrifice of a person who is in disagreement, but commands him to go back from the altar and first be reconciled to his brother, that so God also may be appeased by the prayers of a peace-maker. Our peace and brotherly agreement is the greater sacrifice to God—and a people united in one in the unity of the Father, and of the Son, and of the Holy Spirit.
Source: The Treatises of Cyprian (New Advent)