17 And because we make mention of His passion in all sacrifices (for the Lord's passion is the sacrifice which we offer), we ought to do nothing else than what He did. For Scripture says, “For as often as you eat this bread and drink this cup, you do show forth the Lord's death till He come.” As often, therefore, as we offer the cup in commemoration of the Lord and of His passion, let us do what it is known the Lord did. And let this conclusion be reached, dearest brother: if from among our predecessors any have either by ignorance or simplicity not observed and kept this which the Lord by His example and teaching has instructed us to do, he may, by the mercy of the Lord, have pardon granted to his simplicity. But we cannot be pardoned who are now admonished and instructed by the Lord to offer the cup of the Lord mingled with wine according to what the Lord offered, and to direct letters to our colleagues also about this, so that the evangelical law and the Lord's tradition may be everywhere kept, and there be no departure from what Christ both taught and did.
Source: The Epistles of Cyprian (New Advent)