Some Protestants challenge the notion that Peter is the rock by appealing to the distinction that the Greek text makes between the word for Simon’s new name (Petros) and the rock (petra). For example, Protestant apologist James McCarthy argues:
Why did not the Holy Spirit just repeat the word petros? . . . Then Matthew 16:18 would read, “You are Peter (Petros), and upon this rock (petros) I will build my church.”
From this distinction, apologists who make this argument reason that the difference in words must mean they refer to different things—Petros to Peter and petra to something other than Peter. McCarthy concludes:
When the Holy Spirit inspired the Greek text of the New Testament, He made a distinction between Peter (Petros) and the rock (petra).
Protestant apologist Ron Rhodes concurs with McCarthy, writing:
“Peter” (petros) is a masculine singular term, and “rock” (petra) is a feminine singular term. Hence, they do not have the same referent. Jesus did not say to Peter, “You are Petros and on this Petros I will build my church.” Jesus said, “You are Petros (Peter), and upon this petra, I will build my church.”
Does the mere distinction between the words Petros and petra give grounds to think the rock is not Peter?
With regard to the word Petros, the maleness of Peter provides grounds for why it’s used. As Ron Rhodes points out above, the two words differ in gender: Petros is masculine, and petra is feminine. It would have been unthinkable, at least for first-century Jews, to use a Greek feminine noun for the proper name of a man. Evangelical bible scholar D.A. Carson writes, “In Greek the feminine petra could not very well serve as a masculine name.” New Testament Protestant scholar R.T. France concurs: “The reason for the different Greek form is simply that Peter, as a man, needs a masculine name, and so the form Petros has been coined.”
Now, as to why Matthew doesn’t use Petros in the second instance but rather uses petra, there are a few possible reasons. One is that petra, when compared to Petros, would have been the more familiar term for “rock” to Matthew’s audience. And since the word is being used as a metaphor and not a proper masculine name, petra would have been the natural word for Matthew to use.
R.T. France comments on this lack of familiarity with Petros among Matthew’s readers, writing:
The masculine noun petros occurs infrequently in classical poetic Greek to mean a stone (i.e., a broken piece of rock), though the distinction from petra is not consistently observed. But petros as a common noun is unlikely to have been familiar to Matthew’s readers, as it is not found in the LXX [the Septuagint] (except twice in 2 Maccabees) or in the NT and related literature.
Another possible reason for why Matthew uses petra instead of Petros is for stylistic purposes. Matthew may have wanted to avoid repetition and not use the same word twice. This is not foreign to our own experience. Whenever we write something, like a book, an essay, an article, etc., we avoid using the same words over and over and over again (the repetition is a bit annoying, isn’t it?). That’s why we use thesauruses.
We even do this in speech. If you’re talking to me about your friend John, you’re not going to keep referring to him with language like John walks to John’s house when John leaves John’s job. Rather, you’ll mix it up with the pronouns he and his.
Similarly, it’s possible Matthew didn’t want to be repetitive. As such, he uses two different words that have the same meaning. And the mere fact that Matthew uses two different words no more means he’s referring to two different things than my use of rock and stone means I’m referring to two different things other than the single rock I have in my hand.
Here’s another possible explanation. By using petra, Matthew might have intended to keep a connection with other teachings from Jesus. In Matthew 7:24ff, Matthew records Jesus’ parable of the wise man who “built his house upon the rock [Greek, petra]” (v. 24), and when the winds blew and the floods came, the house did not fall, “because it had been founded on the rock [Greek, petra]” (v. 25). Perhaps Matthew intended to echo the parable: Jesus is the wise man building his house, the Church, on the rock (petra), which is Peter. As R.T. France notes:
[Peter] is to be a “Rock.” And one important function of a rock, as [Matt.] 7:24-27 has reminded us, is to provide a firm foundation for a building. So, on this rock Jesus will build his church, and it will be forever secure.
Another possible reason for the use of different words is to preserve the conceptual distinction between a proper noun (Rock as a proper name) and a common noun (rock as image). Petros in the phrase “you are Petros,” is used as a proper name. Petra in the phrase “upon this petra,” is used as an image. If Matthew would have used Petros in the second instance, it would have read, “You are Peter, and upon this Peter I will build church.” It would have been a bit awkward to use a proper name this way. Using a common noun in the second instance is more natural.
And so, given that we can provide plausible reasons as to why there might be a difference in words without denying that the rock refers to Peter, the argument that Peter is not the rock based on the fact that Petros and petra are different words fails.