rock: solid living rock, whether bedrock, a rock face, or a standing mass of stone, as distinct from a loose stone one could lift or throw
This word names solid, immovable rock: the bedrock under the ground, a rock face, a standing mass of stone. That is what sets it apart from the ordinary stone (λίθος), a single piece a person can pick up, build with, or throw. Rock in this sense is the living rock itself, not a piece broken off from it.
Because such rock was the most stable thing people knew, the word became a natural picture of security and permanence. A house dug down to rock stands through a flood; seed that lands on a shelf of rock sprouts fast and dies for lack of soil; and in figurative speech the rock can be the sure base the church is built on, or a title for Christ, who gave his people water in the desert.
That same hardness makes rock an image of something unyielding that people crash into. Where Christ is set like a rock in Israel's path, some build their lives on him and others trip over him. In those two stumbling passages the word stands in step with the ordinary word for a stone rather than pointing to bedrock, a useful reminder that the difference between the two words is a strong tendency and not an iron rule.
Nuances
What "this rock" refers to in the saying "you are Peter, and on this rock I will build my church" (Matthew 16:18), where the name Jesus gives Simon and the word for rock echo each other.
- majorityThe rock is Peter himself, and specifically Peter as the apostle who has just confessed Jesus as the Messiah: the demonstrative "this" points back to the name just given, the two clauses are joined rather than set against each other, and the feminine noun could not double as a man's name. Several who hold this view stress that it says nothing about successors or a later office.; Oscar Cullmann, "πέτρα, Πέτρος," in Theological Dictionary of the New Testament, vol. 6; W. D. Davies and Dale C. Allison, A Critical and Exegetical Commentary on the Gospel according to Saint Matthew (ICC), vol. 2; Ulrich Luz, Matthew 8-20 (Hermeneia); R. T. France, The Gospel of Matthew (NICNT, 2007); Donald A. Hagner, Matthew 14-28 (Word Biblical Commentary 33B)
- minorityThe rock is the content of Peter's confession, that Jesus is the Messiah, rather than Peter as a person: ordinary Greek usage of "on this rock" points to something other than the person being addressed, and Matthew could simply have written "on you." This was the common reading among the Reformers.; Chrys C. Caragounis, Peter and the Rock (BZNW 58; Berlin: De Gruyter, 1990)
- minorityThe rock is Christ himself, the one foundation the church is built on, with Peter's own letter about Christ as the living stone cited in support.; Augustine, Retractationes 1.21
Disputed
Whether the older Greek difference between this feminine noun for living rock and the matching masculine noun for a stone that can be thrown was still in force in first-century Greek, and whether it can bear any weight in reading Matthew 16:18.
- majorityThe contrast had largely eroded by the first century. The masculine noun had dropped out of everyday use and survives in the New Testament only as Simon's name; λίθος carries ordinary "stone"; and the shift of words in Matthew 16:18 is explained by the need for a masculine form to serve as a man's name. No exegetical argument can rest on the word choice alone.; Oscar Cullmann, "πέτρα, Πέτρος," in Theological Dictionary of the New Testament, vol. 6; BDAG, s.v. πέτρα; D. A. Carson, Matthew, in The Expositor's Bible Commentary
- minorityA residual difference remained in usage, the masculine form still tending toward a stone and the feminine being the ordinary word for rock, though on its own this settles nothing about Matthew 16:18.; Herman N. Ridderbos, Matthew (Bible Student's Commentary)