The Vatican, as a sovereign entity, has its own flag. The yellow-and-white background features a coat of arms, which includes two keys. One key is silver, the other is gold, and they are crossed in front of a crown with three layers.
The symbol of the key is found frequently in the Bible as a sign of authority, specifically in Matthew 16:19, where Jesus said to St. Peter, “I will give you the keys to the kingdom of heaven. Whatever you bind on earth shall be bound in heaven; and whatever you loose on earth shall be loosed in heaven.”
Catholics understand this passage to mean that in using the image of keys, Jesus was invoking the words of Isaiah 22:22, where Eliakim, a humble servant, was given the keys to the House of David. In Biblical times, the king’s steward wore the keys to the palace on his shoulder, and was trusted with opening and closing the gates. All that the keys unlocked belonged to the king, but the steward was trusted with its safekeeping. With His words in Matthew, Jesus was establishing Peter and those who would follow him as stewards of the Church.
The keys on the Vatican flag are visual reminders of this Biblical passage. The golden key represents Peter’s and his successors’ power to bind and loose in heaven, and the silver key represents authority on earth.