Nicene and Post-Nicene Fathers, Vol. II:
On Christian Doctrine: Chapter 14

Chapter 14.—Error of Those Who Think that There is No Absolute Right and Wrong.

22.  But when men unacquainted with other modes of life than their own meet with the record of such actions, unless they are restrained by authority, they look upon them as sins, and do not consider that their own customs either in regard to marriage, or feasts, or dress, or the other necessities and adornments of human life, appear sinful to the people of other nations and other times.  And, distracted by this endless variety of customs, some who were half asleep (as I may say)—that is, who were neither sunk in the deep sleep of folly, nor were able to awake into the light of wisdom—have thought that there was no such thing as absolute right, p. 563 but that every nation took its own custom for right; and that, since every nation has a different custom, and right must remain unchangeable, it becomes manifest that there is no such thing as right at all.  Such men did not perceive, to take only one example, that the precept, “Whatsoever ye would that men should do to you, do ye even so to them,” 1866 cannot be altered by any diversity of national customs.  And this precept, when it is referred to the love of God, destroys all vices when to the love of one’s neighbor, puts an end to all crimes.  For no one is willing to defile his own dwelling; he ought not, therefore, to defile the dwelling of God, that is, himself.  And no one wishes an injury to be done him by another; he himself, therefore, ought not to do injury to another.

Footnotes

Matt. 7.12.  Comp. Tobit 4.15.