“Tantum ergo” is the incipit of the last two verses of Pange lingua, a medieval Latin hymn composed by St Thomas Aquinas circa A.D. 1264. The hymn’s Latin incipit literally translates to “Therefore so great”.
The singing of the Tantum ergo occurs during adoration and benediction of the Blessed Sacrament in the Catholic Church.
Tantum ergo sacramentum
Veneremur cernui,
et antiquum documentum
novo cedat ritui.
Præstet fides supplementum
sensuum defectui.
Genitori genitoque
laus et jubilatio,
Salus, honor, virtus quoque
sit et benedictio!
Procedenti ab utroque
compar sit laudatio!
Amen.