The Greek word for ‘rock’ is petros, and this is the name Jesus gives Simon in Matt 16:18. But a few words later in this same verse Jesus uses the word petra, also meaning ‘rock’, to refer to the ‘rock’ on which He will build His Church. Jesus is obviously punning on the name He has just given Simon. So why not call Simon Petra instead of Petros? One objection is that petra is feminine whereas petros is masculine. But perhaps a more valid one is that there is also some semantic difference between the two words. Petros refers to a single rock or stone whereas petra usually refers to a rock mass. So in Matt 7:24, where Jesus uses the comparison of a man building a house on rock (as opposed to sand), the word used for ‘rock’ is petra, since a house is more likely to be built on a foundation of bed-rock than on a single rock. So likewise in Luke 8:6 the ‘stony ground’ on which the sower sowed his seed is petra, not petros. See also Matt 27:60 (tomb hewn out of ‘rock’ = petra). To ‘petrify’ is, literally, to turn someone or something to stone; figuratively, to terrify a person into immobility, like a stone.
A Word in Season