Ask a British choral singer to name a piece of church music, and Zadok the Priest is a near certain answer, which is a little odd given how narrow its official job description actually is. It exists to be sung at exactly one kind of event, a coronation, that happens perhaps once in a generation. Everywhere else it appears, football tournaments, graduation ceremonies, film soundtracks, it is technically off duty, borrowed for its sound rather than its function.
That borrowing is easy to understand on listening. The piece opens with close to ninety seconds of pure orchestral buildup before the eight part choir enters together at full volume, a structural trick that turns a liturgical cue into one of the most recognizable entrances in choral music. Handel wrote it as one of four anthems for the same 1727 coronation, but the other three, The King Shall Rejoice, My Heart Is Inditing and Let Thy Hand Be Strengthened, never achieved anything close to the same independent life outside the ceremony that produced them.
The Church of England's own coronation liturgy is what keeps the piece anchored to its original purpose, placing it at the single most theologically weighted moment of the rite, immediately before the sovereign is anointed. Everything else the anthem has become, patriotic standard, popular culture reference, football chant tune, sits downstream of that one specific, still functioning liturgical assignment.