Can anyone suggest a good book on the secret codes and identifiers used by the early Christians? (For example, Ixthus=Jesus, or the “Rotas Opera Tenet Arepo Sator” magic square)?
-Bena
I wasn’t aware that the “Rotas” square had any significance, other than being an interesting bit of wordplay. How does it relate to Christianity?
The Latin meaning of the phrase (“Arepo the sower guides the wheel with care,” IIRC) was interpreted to be a description of God carefully guiding the affairs of the world. The letters of the square can also be arranged into a cross of the word “PATERNOSTER” running vertically and horizontally, bracketed by an “A” and “O” for alpha and omega.
-Ben
Um, I’d always heard the “ROTAS square” was nothing more than clever word play, and imperfect word play at that – “imperfect” because it was dependent on the made-up name “Arepo” to work – in other words, a name used because it fit in the acrostic, not because it actually was a real name which, IIRC, it was not. Certainly I’ve never heard of anyone named “Arepo” in any other context (not that that means anything, my knowledge in this area being pathetically limited). And couldn’t it just be coincidence that a “paternoster” cross can be made to fit it, especially since you have to add extra "o"s and "a"s to make that work?
I had always heard that the cross itself was a “code” such as you are speaking of, in that Christian Romans would scratch that sign on the entrances of tombs in whic they were meeting, before it was understood to have any significance to others outside the religion. It was only after being used as a “code” by Christians in the know that it was later taken to be itself a “Christian” sign and widely recognized as such. But I have no cite for this, so feel free to disregard it. 
Yowza. Clearly this is the Great Mystic Key to the Universe. The participants in this discussion seem to disagree over whether the origins of the square are ultimately Christian or Mithraic. (Of course, this wouldn’t be the first thing Christians and Mithraists might accuse each other of “borrowing”.) The Manchester Museum seems to accept it as unequivocally Christian. And this Catholic Church in Yorkshire claims that “arepo” means “plough” (or even “cross”) and renders the phrase “The sower (sator) at the plough (arepo - also cross) holds (tenet) with his sacrifice (opera) the wheels (rotas - of destiny or time).” I ain’t no Latin scholar so I can’t really evaluate that.
Of course, now I feel guilty about hijacking Ben’s thread, which I don’t think was intended to become a lenghty and arcane discussion of early Christian (or Mithraic, as the case may be) graffiti. I don’t really have any recommendations regarding the OP–a search on Amazon for “Christian symbolism” turned up a few titles (Understanding Early Christian Art and Jesus Christ, Sun of God: Ancient Cosmology and Early Christian Symbolism, among others) but I can’t pretend to know anything about them or have any knowledge of the subject. I would advise you resort to your local library, or better yet, a college library.
I’m not aware of a complete book on the subject. (I suspect that information is rather scant on the subject, even if folktales are included.)
This article from the Catholic Encyclopedia - Discipline of the Secret discusses the type of knowledge that was hidden, but does not really describe any specific secret “handshakes.”
This article at Catholic Encyclopedia - Fish Symbolism mentions the dove and the anchor in passing with links to a few other articles.