A reliquary is a container for the physical remains of a saint, or for objects that touched them, and it is frequently the most valuable thing a church possesses.
Housing a presence
The relic is held to make the holy person present and accessible, a point at which heaven touches earth, and the reliquary exists to protect, honour and display that presence, which is why so much wealth was lavished on it. Reliquaries take the form of caskets, of busts or arms shaped to the relic within, of jewelled crosses, or of small containers worn on the body, and the finest are among the supreme works of medieval metalwork. The altar of a Catholic church has traditionally contained a relic, which ties the reliquary to the very structure of the building.
Devotion, doubt and parallels
The cult of relics drove much of medieval pilgrimage and was a major object of the Reformation critique, which attacked both the veneration and the widespread trade in dubious relics, so the reliquary sits at the centre of a long argument about the material mediation of the holy. The impulse is not confined to Christianity: Buddhism enshrines the relics of the Buddha and of revered teachers in stupas, and Islam preserves relics associated with the Prophet, so the reliquary has close parallels in the containers other traditions build to house what remains of their holy dead.