A correspondent writes:
"I”ve read the article [on catholic.com] on Peter the Rock and have a few questions. You make a distinction between Attic and Koine Greek and state that the New Testament was written in Koine Greek and because of this there is no difference in the meaning of petros vs. petra. Is this the only passage in the New Testament for which this applies? I was speaking of this to a colleague of mine who has studied Greek, although not extensively, and she informed me that the explanation for the difference is due to the placement of the word within the sentence. Since Greek is a reflexive language, the meaning stays the same even though the form is changed. Is this correct? Also, to which resource would one go to substantiate the claim regarding the Attic vs. Koine Greek as applied to the Bible?"
Greek is an inflected (not "reflexive") language, which means that the forms of nouns change based on the function a word is performing in a sentence. When this happens, the base meaning of the word remains the same. The inflection communicates information about how the word is being used grammatically but not what it means.
In the case of petros vs. petra, the change is not an inflection. Petros and petra are two different words in Greek. They are similar because they are cognates (just as "president" and "presider" are cognates in English but are nonetheless two different words with different, though related, meanings). Because they are two different words, the inflection (change of form) of petros and petra is not what is at issue here. The basic meanings of the terms is.
The point the article is making is that in Attic Greek there was a slight difference in meaning between the two, but in Koine Greek (the dialect of the New Testament) they were synonyms. A place to look this up is D. A. Carson’s commentary on Matthew 16 in the Expositors Bible Commentary. He makes this point very well, and he is a highly-respected Evangelical Bible scholar.