Innocent III on Mysterium Fidei

From Denzinger, Sources of Catholic Dogma

p.163 The Form of the Eucharistic Sacrament and its Elements
[from the letter "Cum Marthae circa", November 29, 1202 to John, Archbishop of Lyons]

"You have asked (indeed) who added to the form of words which Christ Himself expressed when He changed the bread and wine into the Body and Blood, which are in the Canon of the Mass that the general Church uses, but which we find expressed by none of the Evangelists . . . In the Canon of the Mass that expression, "mysterium fidei," is found interspersed among His words . . . Surely we find many such things omitted by the Evangelists from the words as well as from the deeds of the Lord . . . Therefore we believe that the form of words, as is found in the Canon, the Apostles received from Christ, and their successors from them."

Innocent III (1198 - 1216)