I was just thinking about this the other day. If this event is so important in the founding of the Church, why does it only appear in 1 of the 4 gospels? Mark and Luke have the account of the interaction, however their accounts lack any mention of rocks, keys, binding or loosing. If this event was the founding of Christ's Church on Earth, why is it only mentioned once? The event is not even recorded in John's gospel. When you read the conversation in the 3 gospels where it is recorded, the recurring theme is for the apostles not to reveal Jesus as the Messiah.