for Christians of many traditions, the sanctus, trisagion or qeduššah, is a familiar feature in the eucharistic prayer. It finds a place in almost every classical anaphora, and it has generally been regarded in modern liturgical revision as a necessary constituent of the eucharistic prayer. Its ultimate written source is not a mystery. It is adapted from the song of the seraphim of Isaiah 6:3 which, in different form and on the lips of four living creatures (from Ezekiel), recurs in Revelation 4:8. However, from the viewpoint of the history and theology of Christian liturgy, its occurrence within the eucharistic prayer remains something of a mystery. Why did this biblical song come to be inserted within a prayer in which the church follows the example and command of Jesus at the Last Supper? There is no suggestion in the Gospels that Jesus uttered the sanctus at this meal and enjoined its recitation, nor does there appear to be any evidence to suggest that it was ever a recognised constituent of Jewish meal prayers.

Although in practically all post-fourth-century classical anaphoras, East and West, the sanctus occurs at some point in the first part of the prayer, it is noticeably absent from some significant early texts, namely the Apostolic Tradition attributed to Hippolytus, Testamentum Domini, from the anaphora of Epiphanius and, according to M. A. Smith, an anaphora alluded to in Narsai's Homily XVII.