Held here as a hymn, the Trisagion, Holy God, Holy Mighty, Holy Immortal, have mercy on us, is sung near the beginning of the Byzantine Divine Liturgy and throughout Eastern Christian worship. Its threefold address to God as holy, mighty and immortal is distinct from the Sanctus of the eucharistic prayer, and the Eastern tradition preserves an account of its origin in a hymn revealed during an earthquake at Constantinople, when a child was said to have been caught up and heard the angels singing it. A single addition to the hymn, the phrase confessing that the holy and immortal one was crucified, became a serious point of controversy in the fifth and sixth centuries, entangled with disputes over the union of the divine and human in Christ, since to say that God was crucified touched directly on how the natures of Christ were understood. Sung repeatedly and with reverence, the Trisagion is among the most cherished hymns of the Orthodox liturgy and is used also in the Western church, notably in the Good Friday reproaches.
The added phrase who was crucified for us, associated with the Miaphysite patriarch Peter the Fuller of Antioch in the fifth century, was rejected by those who held to the Council of Chalcedon as implying that the divine nature itself had suffered, and disputes over whether to sing the hymn with or without the addition became a visible marker of the deep christological split between the Chalcedonian and non-Chalcedonian churches that persists to this day. In the Byzantine rite the hymn is sung with a slow processional character, underscoring its place as one of the most theologically weighted moments of the liturgy.