The Keys of Heaven
Christianity · 1st century CE onward
ca. 1100 · Byzantine · gold, silver and enamel, cloisonné
The Metropolitan Museum of Art, Gift of J. Pierpont Morgan, 1917 · CC0
Peter, to whom the keys were given. This particular medallion shows him holding a cross-staff rather than the keys themselves.
Matthew has Christ hand Peter "the keys of the kingdom of heaven", meaning the authority to bind and to loose. Authority is difficult to paint. Keys are not — and so within a few centuries the metaphor had hardened into hardware: two of them, gold and silver, crossed, on the arms of the papacy and in the hand of every painted Peter since. It is the clearest case in the record of an abstraction becoming an object because an object could be held up.
A key that opens a gate is the oldest form this product takes. Ours is cut from brass and there are two of them.
How we know Scriptural, then pictorial — the keys are named in Matthew and drawn as objects ever after.