14 There is then no reason, dearest brother, for any one to think that the custom of certain persons is to be followed, who have thought in you past that water alone should be offered in the cup of the Lord. For we must inquire whom they themselves have followed. For if in the sacrifice which Christ offered none is to be followed but Christ, assuredly it behooves us to obey and do that which Christ did, and what He commanded to be done, since He Himself says in the Gospel, “If you do whatsoever I command you, henceforth I call you not servants, but friends.” And that Christ alone ought to be heard, the Father also testifies from heaven, saying, “This is my well-beloved Son, in whom I am well pleased; hear Him.” Wherefore, if Christ alone must be heard, we ought not to give heed to what another before us may have I thought was to be done, but what Christ, who is before all, first did. Neither is it becoming to follow the practice of man, but the truth of God; since God speaks by Isaiah the prophet, and says, “In vain do they worship me, teaching the commandments and doctrines of men.” And again the Lord in the Gospel repeals this same saying, and says, “You reject the commandment of God, that you may keep your own tradition.” Moreover, in another place He establishes it, saying, “Whosoever shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven.” But if we may not break even the least of the Lord's commandments, how much rather is it forbidden to infringe such important ones, so great, so pertaining to the very sacrament of our Lord's passion and our own redemption, or to change it by human tradition into anything else than what was divinely appointed! For if Jesus Christ, our Lord and God, is Himself the chief priest of God the Father, and has first offered Himself a sacrifice to the Father, and has commanded this to be done in commemoration of Himself, certainly that priest truly discharges the office of Christ, who imitates that which Christ did; and he then offers a true and full sacrifice in the Church to God the Father, when he proceeds to offer it according to what he sees Christ Himself to have offered.
15.But the discipline of all religion and truth is overturned, unless what is spiritually prescribed be faithfully observed; unless indeed any one should fear in the morning sacrifices, lest by the taste of wine he should be redolent of the blood of Christ. Therefore thus the brotherhood is beginning even to be kept back from the passion of Christ in persecutions, by learning in the offerings to be disturbed concerning His blood and His blood-shedding. Moreover, however, the Lord says in the Gospel, “Whosoever shall be ashamed of me, of him shall the Son of man be ashamed.” And the apostle also speaks, saying, “If I pleased men, I should not be the servant of Christ.” But how can we shed our blood for Christ, who blush to drink the blood of Christ?
Source: The Epistles of Cyprian (New Advent)