Now when the the gospel of Matthew was written in Greek, either by the author himself, or by a later translator (in fact, there is much evidence from the early church to support an Aramaic original; Papias, Irenaeus, Origen, Eusebius, Chrysotom, Epiphanius, Augustine), an effort was made to preserve the pun so apparent in the Aramaic language. There was one problem though that the Aramaic language didn't have, as well as other languages such as English and French. In Aramaic, the verse would read: You are Kepha and upon this kepha. Greek nouns have gender specific endings though. So in verse 18, when the writer needed to replace each 'kepha' with a Greek equivalent, he ran into a problem. The word 'petra' could be used for the second appearance of kepha in the verse, but because 'petra' is feminine in gender, it couldn't be used for a man's name. In order to do this, a masculine ending had to be placed on it, and this is where 'petros' comes about. As I said before, many languages don't have this problem as can be seen by a rendering of this verse in each language:
French: Tu es Pierre et sur cette pierre
Old Syriac: Anath-her Kipha, v'all hode kipha
http://www.geocities.com/orthopapism/kipha.htmlGreek doesn't merely have gender changes. Each word is a state of being. Some words are fem. some masculine - just as in German or Japanese. If you change it's gender, it effects the outcome of the meaning. I'm sure you are aware of the consequence of this in Japanese. In earlier times, a simple mistake in gender would be an insult but to the wrong person would also result in a quick parting of the head from the body without so much as a moments notice for one to bid goodbye to the other.
So, finally, the definitions from Strongs:
4073 petra petra, pet'-ra fem. of the same as 4074; a (mass of) rock (lit. or fig.) -rock
4074 petros petros pet'-ros; appar. a prim. word; a (piece of) rock (larger than 3037); as a name, Petrus, an apostle: - Peter, rock. Comp. 2786
2786 khfas kephas kay-fas' of chald. or. [comp 3710]; the Rock; Cephas (ie. Kepha), a surname of Peter: - Cephas.
3710 - Keph, kafe; from 3721; a hollow rock: - rock.
3721 - kaphaph, kaw-faf' a prim. root; to curve: - bow down (self).
petros, as it has been proffered, was not merely changed from petra to make a name. It is a primitive word unto itself that predates the apostle and the idea of use as a name. The same crowd that argues it was changed also want to argue that the word prior to Jesus time had become interchangeable with the word lithos. I guess the two bits of scholarship don't know when to shut up when raining on one another's parades. But I'm used to this. Whatever is said this moment may be discarded later if it becomes inconvenient to a more important thing - image. LOL.