The Trisagion, sometimes called by its opening line Agios O Theos, is a standard hymn of the Divine Liturgy in most of the Eastern Orthodox, Western Orthodox, Oriental Orthodox, and Eastern Catholic churches.
Old Testament Trinity icon by Andrei Rublev, c. 1400 (Tretyakov Gallery, Moscow)
An Icon of Christ the Ancient of Days fresco Ubisi, Georgia
Divine Liturgy or Holy Liturgy is the usual name used in most Eastern Christian rites for the Eucharistic service.
Icon of Ss. Basil the Great (left) and John Chrysostom, ascribed authors of the two most frequently used Eastern Orthodox Divine Liturgies, c. 1150 (mosaic in the Palatine Chapel, Palermo).
Church of Saint Sava, Christmas, Belgrade, 7 January 2021
Mass of Paul VI (post–Vatican II)
Tridentine Mass (1962 missal), Solemn Mass form