Petros

Before it was anyone's name, this was an ordinary Greek word for a stone or a lump of rock. Greek speakers did not use it as a personal name, and no clear case of a man called by it turns up before this one. So when the Gospels attach it to Simon, the word has not gone quiet: it still sounds like a description, something close to calling a man "Rock."

The name arrives in the story as a gift rather than an inheritance. At their first meeting Jesus tells Simon he will be called Cephas (Κηφᾶς), the Aramaic word for a rock, "which is translated" by this Greek name (John 1:42). Mark and Luke record the giving plainly, inside the list of the twelve. Matthew puts it at the high point of the confession near Caesarea Philippi, where the new name and the promise about building are spoken in one breath. After that the writers mostly just use it, the way any name is used, and his birth name (Σίμων) often stands beside it.

Matthew 16:18 sets the new name next to the ordinary word for bedrock (πέτρα) in the same sentence, and a long argument has grown up over the switch. Older writers made much of it, treating the name as a small loose stone and the other word as a cliff. Most lexicographers now read the change as a matter of grammar and naming rather than size: a man's name wanted the masculine form, and the Aramaic sentence behind it would have used one word twice.

Paul is the odd one out. In his letters he nearly always calls the man by the Aramaic form, keeping the Greek name for one short stretch of Galatians, which is why the same apostle can look like two people to an English reader moving between the Gospels and the letters.