The extent and character of Gregory's works in connexion with the liturgy and the music of the church is a subject of dispute.
But though it is certain that he introduced three changes in the liturgy itself (viz.
There is a liturgy which bears his name, and which exists in two forms; the one form was found in a MS. of the 12th century in Calabria, and is, according to Renaudot, the foundation of the three liturgies of St Basil, St Gregory Nazianzen and St Cyril; the other is that which is used by the Maronite and Jacobite Syrians.
Mission work commenced in Bulgaria during the latter part of the 9th century; thence it extended to Moravia, where in 863 two Greek missionaries - Cyril and Methodius - provided for the people a Slavonic Bible and a Slavonic Liturgy; thence to Bohemia and Poland, and so onwards to the Russian kingdom of Ruric the Northman, where about the close of the 10th century the Eastern Church " silently and almost unconsciously bore into the world her mightiest offspring."
(10) The Congregation of Rites (Congregatio sacrorum Rituum), founded by Sixtus V., has exclusive charge of the liturgy and liturgical books; it also deals with the proceedings in the beatification and canonization of saints.