19 Therefore it befits our religion, and our fear, and the place itself, and the office of our priesthood, dearest brother, in mixing and offering the cup of the Lord, to keep the truth of the Lord's tradition, and, on the warning of the Lord, to correct that which seems with some to have been erroneous; so that when He shall begin to come in His brightness and heavenly majesty, He may find that we keep what He admonished us; that we observe what He taught; that we do what He did. I bid you, dearest brother, ever heartily farewell.
Source: The Epistles of Cyprian (New Advent)