Byzantine tradition traces the Trisagion's revelation to a specific legendary event, an earthquake that struck Constantinople under Theodosius II in the fifth century, during which a child was reportedly caught up into the sky and returned reciting these words as what he had heard the angels sing, after which the assembled people added eleison hemas, have mercy on us, and the earthquake ceased. Byzantine chronicles record this account as the traditional origin of the formula's introduction into the liturgy of the capital, though the legend functions to authorize an already spreading practice as much as to document how it actually began.
The Trisagion is frequently confused with the Sanctus, the holy, holy, holy sung within the eucharistic prayer in both Eastern and Western liturgies and drawn directly from the seraphim's threefold acclamation in Isaiah 6:3, but the two are textually distinct formulas serving different liturgical moments: the Sanctus repeats holy three times before naming God as Lord of hosts, following Isaiah's wording closely, while the Trisagion instead names three separate attributes, holy God, holy mighty one, holy immortal one, each occurring once rather than one word repeated three times, and is sung earlier in the Divine Liturgy, at the entrance, rather than within the eucharistic prayer itself. Both ultimately echo Isaiah's vision, but Eastern liturgical scholarship treats them as two separate compositions inspired by a common biblical source rather than as one text existing in two versions.