...Protestant argument revolves around the idea that there are two words for "Rock" in the Greek language: petra and petros, and that the former refers to a big boulder while the latter refers to a small pebble. They claim that because the Greek rendering of Matthew's Gospel uses the word "petros," that Jesus was playing down Peter's significance. Petros and petra meant exactly the same thing at the time Matthew was translated into or written in Greek, as Greek literature attests (lithos is the word for "little stone"). Petros was chosen because it's a masculine noun and Peter was a man. If you had High School French, here's an analogy for you: even if petros and petra had different meanings in the Koine Greek spoken at the time of Christ (which isn't so), the use of the masculine form, petros, by the Greek translator of Matthew would have made sense anyway. Say you were wanting to refer to a man metaphorically as "a portal" and were wanting to give him a nickname that reflected that. In French, you could call him "la porte," a feminine noun meaning door, or "le portail," a masculine noun meaning gate. It'd make sense to use the masculine noun even though "gate" is a smaller thing than "door." At any rate, "big rock" or "little rock," rock is rock and Christ said THOU art "Rock" -- and Jesus was not speaking Greek, but Aramaic and used the word "Kepha"; this is why Simon Peter is most often called "Cephas..."
Now you know the rest of the story...