PHB, page 151
Spellcasting Focus Varies
A holy symbol is a representation of a god or pantheon. It might be an amulet depicting a symbol representing a deity, the same symbol carefully engraved or inlaid as an emblem on a shield, or a tiny box holding a fragment of a sacred relic. A cleric or paladin can use a holy symbol as a spell-casting focus. To use the symbol in this way, the caster must hold it in hand, wear it visibly, or bear it on a shield. Multiple variations of this item exist, as listed below:

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