Wrong is wrong, even if everyone is doing it. Right is right, even if no one is doing it.
Attributed to St. Augustine (and also to William Penn, but not recently on Facebook that I’ve seen). It’s actually a modification of a quotation from G. K. Chesterton:
Right is right, even if nobody does it. Wrong is wrong even if everybody is wrong about it.
Illustrated London News, May 11, 1907, or Collected Works, 27:463.
It’s not impossible that St. Augustine and/or William Penn said it and GKC took it from one of them, but I can’t find any evidence for that.
St. Augustine’s name seems to have been attached to it only recently. The earliest Google Book reference I can find is 2002.
I did find one person who claims it’s a summary of Ennarations on the Psalms 145:7. Here’s what that passage says:
Right and wrong are contraries. Right is what is just. For not all that is called right, is right. What if a man lay down for you unjust right? Nor indeed is it to be called right, if it is unjust. That is true right, which is also just.
Try as I might, I can’t see that as saying what the alleged quotation says (though it does contain “right is right” as long as you don’t mind an intervening comma.)
I have also been told it’s in City of God. That’s a loooooong book, but I can say that the phrase “right is right” does not appear in the version on Project Gutenberg (and yes, I checked both volumes).
I’m standing by my verdict.