16:18 Peter. The Lord is here making a play on words; in the Greek, “Peter” is petros, meaning a small stone, whereas “rock” is petra, meaning a great rock mass, solid and immovable. Even if Jesus was speaking in Aramaic, in which both meanings are expressed simply by Cephas, He was making a distinction between the two (perhaps by gestures) which Matthew picked up and (under divine inspiration) translated by the two different Greek words.
The massive rock foundation on which Christ would build His Church was Peter’s great confession of Jesus as the Creator and the Son of the living God. Peter (representing the twelve and, indeed, all who would make the same confession) would be living stones in the church built on the foundation of such confession (I Peter 2:5; Ephesians 2:19-22).