Are you really asking me to tell you why God makes the choices that God makes? That seems a little above the pay grade of any man to explain the mind of God.
Best guess, because Peter was a rock ... fearless in Acts 2 and a leader after Jesus restored him. However, the CHURCH is not built on the strength of Peter (the boulder) but the immovable bedrock of the death and resurrection of Jesus Christ (God Incarnate).
You wrote, "the bedrock was the truth that Jesus is the Christ, son of the living God that Peter had just spoken." This does not make sense for two reasons:
1. Peter was not the first to acknowledge and confess that Jesus is the Christ, the Son of the Living God. Peter's brother, Andrew, along with Nathaniel were the first. Yet Christ does not change their names.
2. A mere seven verses after Peter's confession, Jesus reprimands him for denying the very mission of the Christ. In other words, his confession was short lived and not an "immovable boulder".
Hence, it must be the
PERSON whom Christ calls the rock. Here are Protestant commentators acknowledging this...
The meaning is, 'You are Peter, that is Rock, and upon this rock, that is, on you, Peter I will build my church.' Our Lord, speaking Aramaic, probably said, “And I say to you, you are Kepha, and on this kepha I will build my church.”
Jesus, then, is promising Peter that he is going to build his church on him! I accept this view." - William Hendriksen, member of the Reformed Christian Church, Professor of New Testament Literature at Calvin Seminary, New Testament Commentary: Exposition of the Gospel According to Matthew (Grand Rapids, MI: Baker, 1973), page 647
"Nowadays a broad consensus has emerged which — in accordance with the words of the text —
applies the promise to Peter as a person. On this point liberal (H. J. Holtzmann, E. Schweiger) and conservative (Cullmann, Flew) theologians agree, as well as representatives of Roman Catholic exegesis." - Gerhard Maier, Evangelical Lutheran theologian, "The Church in the Gospel of Matthew: Hermeneutical Analysis of the Current Debate” (Flemington Markets, NSW: Paternoster Press, 1984), page 58
"Although it is true that petros and petra can mean “stone” and “rock” respectively in earlier Greek, the distinction is largely confined to poetry. Moreover the underlying Aramaic is in this case unquestionable; and most probably kepha was used in both clauses (“you are kepha” and “on this kepha”), since the word was used both for a name and for a “rock”. The Peshitta (written in Syriac, a language cognate with Aramaic) makes no distinction between the words in the two clauses.
The Greek makes the distinction between petros and petra simply because it is trying to preserve the pun, and in Greek the feminine petra could not very well serve as a masculine name." - Donald A. Carson III, Baptist and Professor of New Testament at Trinity Evangelical Seminary, The Expositor’s Bible Commentary: Volume 8 (Matthew, Mark, Luke) (Grand Rapids, MI: Zondervan, 1984), page 368
"The word Peter petros, meaning “rock” (Gk 4377), is masculine, and in Jesus’ follow-up statement he uses the feminine word petra (Gk 4376).
On the basis of this change, many have attempted to avoid identifying Peter as the rock on which Jesus builds his church. Yet if it were not for Protestant reactions against extremes of Roman Catholic interpretations, it is doubtful whether many would have taken “rock” to be anything or anyone other than Peter." - Donald A. Carson III, Baptist and Professor of New Testament at Trinity Evangelical Seminary, Zondervan NIV Bible Commentary — New Testament, vol. 2 (Grand Rapids, MI: Zondervan, 1994), page 78
"The Saviour, no doubt, used in both clauses the Aramaic word kepha (hence the Greek Kephas applied to Simon, John i.42; comp. 1 Cor. i.12; iii.22; ix.5; Gal. ii.9), which means rock and is used both as a proper and a common noun....
The proper translation then would be: “Thou art Rock, and upon this rock”, etc.", John Peter Lange, German Protestant scholar and theologian, Lange’s Commentary on the Holy Scriptures: The Gospel According to Matthew, vol. 8 (Grand Rapids, MI: Zondervan, 1976), page 293
Here is your argument specifically addressed...
"Many insist on the distinction between the two Greek words, thou art Petros and on this petra, holding that if the rock had meant Peter, either petros or petra would have been used both times,
and that petros signifies a separate stone or fragment broken off, while petra is the massive rock. But this distinction is almost entirely confined to poetry, the common prose word instead of petros being lithos; nor is the distinction uniformly observed.
But the main answer here is that our Lord undoubtedly spoke Aramaic, which has no known means of making such a distinction [between feminine petra and masculine petros in Greek]. The Peshitta (Western Aramaic) renders, “Thou are kipho, and on this kipho”. The Eastern Aramaic, spoken in Palestine in the time of Christ, must necessarily have said in like manner, “Thou are kepha, and on this kepha”.... Beza called attention to the fact that it is so likewise in French: “Thou art Pierre, and on this pierre”; and Nicholson suggests that we could say, “Thou art Piers (old English for Peter), and on this pier.” - John A. Broadus, Baptist scholar, Commentary on the Gospel of Matthew (Valley Forge, PA: Judson Press, 1886), pages 355-356
"
By the words “this rock” Jesus means not himself, nor his teaching, nor God the Father, nor Peter’s confession, but Peter himself. The phrase is immediately preceded by a direct and emphatic reference to Peter. As Jesus identifies himself as the Builder, the rock on which he builds is most naturally understood as someone (or something) other than Jesus himself. The demonstrative this, whether denoting what is physically close to Jesus or what is literally close in Matthew, more naturally refers to Peter (v. 18) than to the more remote confession (v. 16). The link between the clauses of verse 18 is made yet stronger by the play on words, “You are Peter (Gk. Petros), and on this rock (Gk. petra) I will build my church”. As an apostle, Peter utters the confession of verse 16; as a confessor he receives the designation this rock from Jesus." - J. Knox Chamblin, Presbyterian and New Testament Professor, Reformed Theological Seminary, “Matthew”, Evangelical Commentary on the Bible (Grand Rapids, MI: Baker, 1989), page 742
"
Acknowledging Jesus as The Christ illustrates the appropriateness of Simon's nickname “Peter” (Petros = rock). This is not the first time Simon has been called Peter (cf. John
1:42), but it is certainly the most famous. Jesus’ declaration, “You are Peter”, parallels Peter’s confession, “You are the Christ”, as if to say, “Since you can tell me who I am, I will tell you who you are.” T
he expression “this rock” almost certainly refers to Peter, following immediately after his name, just as the words following “the Christ” in v. 16 applied to Jesus. The play on words in the Greek between Peter’s name (Petros) and the word “rock” (petra) makes sense only if Peter is the rock and if Jesus is about to explain the significance of this identification." - Craig L. Blomberg, Baptist and Professor of New Testament, Denver Seminary, The New American Commentary: Matthew, vol. 22 (Nashville: Broadman, 1992), pages 251-252
"
On this rock I will build my church: the word-play goes back to Aramaic tradition. It is on Peter himself, the confessor of his Messiahship, that Jesus will build the Church. The disciple becomes, as it were, the foundation stone of the community. Attempts to interpret the “rock” as something other than Peter in person (e.g., his faith, the truth revealed to him) are due to Protestant bias, and introduce to the statement a degree of subtlety which is highly unlikely." - David Hill, Presbyterian minister and Senior Lecturer in the Department of Biblical Studies, University of Sheffield, England, “The Gospel of Matthew” The New Century Bible Commentary (London: Marshall, Morgan & Scott, 1972), page 261
"The play on words in verse 18 indicates the Aramaic origin of the passage.
The new name contains a promise. “Simon”, the fluctuating, impulsive disciple, will, by the grace of God, be the “rock” on which God will build the new community." - Suzanne de Dietrich, Presbyterian theologian, The Layman’s Bible Commentary: Matthew, vol. 16 (Atlanta: John Knox Press, 1961), page 93
"The natural reading of the passage, despite the necessary shift from
Petros to petra required by the word play in the Greek (but not the Aramaic, where the same word kepha occurs in both places), is that it is Peter who is the rock upon which the church is to be built....
The frequent attempts that have been made, largely in the past, to deny this in favor of the view that the confession itself is the rock... seem to be largely motivated by Protestant prejudice against a passage that is used by the Roman Catholics to justify the papacy." - Donald A. Hagner, Fuller Theological Seminary, Matthew 14-28, Word Biblical Commentary, vol. 33b (Dallas: Word Books, 1995), page 47
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