In today's gospel, Jesus asks his disciples who do people think that he really is. And only one appears to have the right answer, Peter, who announces, ‘You are the Messiah, the Son of the living God.’ And Jesus is clearly delighted. And, almost like a reward for the correct answer, he turns to Peter tells him, "On this rock I will build my church".
The rock of course is Peter, his name means a rock in Greek. And so it was that Peter became the first Pope and all Popes ever since have had the title, "Successor of the Prince of the Apostles", and the Prince of the Apostles was Peter. It is also said that the Latin word, for the Pope, Papa, was an acronym, representing the phrase "Petri apostoli potestatem accipiens", which means "the one who receives the power of the apostle Peter".
PIcture above: Church built on a rock, in Fuente del Conde