At the conclusion of the Preface comes the Sanctus, which in Latin means “Holy.” The Sanctus, like the Gloria, is intended to be sung – in fact, many different settings of the Latin text exist even in Gregorian chant.
The only textual difference from our current version is that “God of power and might” becomes “God of hosts.” The word “hosts” refers to a great gathering or multitude, and speaks here of God’s command over the heavenly host of angelic armies.
This reference has a Biblical foundation in Isaiah 6:1-3, where the prophet writes, “I saw the Lord seated on a high and lofty throne… Seraphim were stationed above… ‘Holy, holy, holy is the Lord of hosts!’ they cried one to the other. ‘All the earth is filled with his glory!’”
And in Luke 2:13, a “multitude of the heavenly host” also announces the birth of Jesus to the shepherds.
The words of the final three lines of the Sanctus can be found in the Gospel of Matthew, during the entry of Jesus into Jerusalem before His Passion, as the people shouted, “Hosanna to the Son of David; blessed is he who comes in the name of the Lord; hosanna in the highest” (Mt 21:9). Versions of this acclamation appear in the other Gospels, and the “Blessed is he…” line comes from Psalm 118 (117), amidst a passage that became understood as a reference to Christ.
The Sanctus reminds us that all creatures on “heaven and earth” owe thanksgiving to God (“Eucharist” actually means “thanksgiving”). And because we truly believe that the Angels are also present and worshiping with us as we celebrate the Holy Eucharist, then every fiber of our being is made to reflect the utmost reverence. Therefore, immediately after the Sanctus, we kneel. Kneeling is a sign of respect and humility that is distinctly human and bodily – it is something that even the Angels, being pure spirit, cannot do.