...and in Greek, petros and petra (the two words used in the passage) are different; it follows in the context of Peter's great declaration of the foundational Deity of the Lord Jesus Christ. The preaching of the gospel by the Apostles was the key to the kingdom.
Indeed Jesus did say "Petra" referring to the the rock he was building his Church upon, and then "Petrus" referring to Peter, but the difference is masculine and feminine. Just as I wouldn't call someone named Mario "Maria." The words, "Rock" is feminine by default, but since he was addressing peter (masculine) it was "Petrus."