This is a very easy question to answer, and I'm not sure why the obvious hasn't been clearly enough stated: Πέτρος (*Petros*) is simply Greek (naturally, because the New Testament was written in Greek) for the name Jesus *actually* gave him in Aramaic: כיפא (*Kefa*—Rock). In Greek, Kepha is trans*lit*erated (not trans*lat*ed) as Κηφᾶς (*Kephas*—a representation of the Aramaic name with the necessary Greek grammar 'salting'), but is sometimes trans*lat*ed into the Greek, and so you see Πέτρος (*Petros*—the equivalent of כיפא in Greek; Rock, Stone). --- St. Paul might have used *kepha* to emphasize St. Peter's role (as his name signifies) as the Rock or leader of the Church (such as emphasizing that he had to even rebuke the 'Rock' of the Church, when it was called for). We see that Jesus, when He first called Peter (not later in His ministry is he given the new name, as is a common misconception) to be His disciple, gave him a new name. As we know from Scripture, this always signifies a new role or significance for the person: e.g. Abram (*high father*) to Abraham (*father of many*). >**John 1:41** >He findeth first his brother Simon, and saith to him: We have found the Messiah, which is, being interpreted, the Christ. And he brought him to Jesus. And Jesus looking upon him, said: Thou art Simon the son of Jona: thou shalt be called Cephas [still a Greek transliteration of Aramaic *Kepha*], which is interpreted Peter [i.e. which in the Greek in which this is written, is *Petros*]. Then later, Jesus reveals the reason behind the name change: >**Matthew 16:16-19** >Simon Peter answered and said: Thou art Christ, the Son of the living God. And Jesus answering, said to him: Blessed art thou, Simon Bar-Jona [Ar. for 'Simon, son of John']: because flesh and blood hath not revealed it to thee, but my Father who is in heaven. **And I say to thee:** That **thou art *Peter [Petros\*]***; and upon this **rock [petra\*]** ***I will build my church,*** and the gates of hell shall not prevail against it. And **I will give to thee the keys of the kingdom of heaven.** And **whatsoever thou shalt bind** upon earth, it shall be bound also in heaven: and **whatsoever thou shalt loose** on earth, it shall be loosed also in heaven. <sup>\* A name for a man cannot be the bare (feminine) word for rock in Greek (Petra) and so is made into a masculine name form: Petros. It has nothing to do with a big rock and a small rock; such is nonsense and destroys the grammatical connection of 'this' with 'rock'—'Thou art Peter. And in other news, something nothing to do with little rocks, upon this huge cliff I will build by Church.'</sup> He is making Peter the 'rock' upon which His Church is built in the sense that He is entrusting him with the authority of 'steward' in His Kingdom of Heaven on earth. In saying what He does the way He does He harkens back to a passage in Isaiah where the same language is used of Eliakim: >**Isaiah 22:20:-23** >And it shall come to pass in that day, that I will call my servant Eliakim the son of Helcias, and I will clothe him with thy robe, and will strengthen him with thy girdle, and will **give thy power into his hand:** and he shall be as a father to the inhabitants of Jerusalem, and to the house of Juda. **And I will lay the key of the house of David upon his shoulder:** **and he shall open, and none shall shut: and he shall shut, and none shall open.** And *I will fasten him as a peg in a sure place,* and he shall be for a throne of glory to the house of his father. This investment with authority (seen throughout Acts etc) is the sense in which Peter, the exemplar in the profession of the Faith in Christ as Messiah and Lord, the Son of the Highest, is the 'rock' of the Church. The hinge around which the unity of Faith is justly pivoted, per its institution this why by the Lord Jesus Himself. --- St. Paul cleared up any disputes he had with St. Peter as we see in Acts 15.