“Thou art Peter, and upon this rock I will build my church.”
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Why was Peter called “rock” (petros in Greek)?
Because he sank like a rock!
“And when the disciples saw him walking on he sea, they were troubled, saying, It is a spirit; and they cried out for fear. But straightway Jesus spake unto them saying, Be of good cheer; it is I; be not afraid. And Peter answered him and said, Lord, if it be thou, bid me come unto thee on the water. And he said, Come. And when Peter was come down out of the ship, he walked on the water, to go to Jesus. But when he saw the wind boisterous, he was afraid: and beginning to sink, he cried, saying Lord, save me. And immediately Jesus stretched forth his hand, and caught him, and said unto him, O thou of little faith, wherefore didst thou doubt?” Matthew 14:26-31.