The mystery of faith
This phrase appears in the middle of the formula of consecration in the Tridentine Mass.
None of the Institution Narratives have this expression in them.
"The phrase 'mysterium fidei' appears in the earliest written sacramentaries of the Roman Rite, which means only that it was in use no later then the seventh century. It appears in those rites which were heavily influenced by Rome (French Gallican, modern Ambrosian, Maronite) in later years, but nowhere else--not among the Copts, Byzantines, and Armenians, who make up the vast majority of the Eastern Churches both Orthodox and Catholic. It does not appear in the New Testament accounts of the Last Supper."
Some suggest that this was an acclamation made by a deacon to alert the congregation that the consecration had taken place. I'm not completely sold on this idea. It seems more natural to me that "mystery of faith" refers to the Eucharistic elements which have just been consecrated. The sacred species are the mystery (sacramentum) of faith--outward signs of inward realities, effective signs that cause what they signify.
|Simili modo postquam cenatum est,||In like manner, when he had eaten,|
|accipiens et hunc praeclarum calicem||taking also the blessed cup into his|
|in sanctas ac venerabiles manus suas:||holy and venerable hands, and|
|item tibi gratias agens, benedixit,||again giving thanks to You, He blessed it,|
|deditque discipulis suis, dicens:||and gave it to His disciples, saying:|
|Accipite, et bibite ex eo omnes.||Take and drink you all of this:|
|Hic est enim calix sanguinis mei,||For this is the Chalice of my blood,|
|novi et aeterni testamenti:||of the new and everlasting covenant:|
|mysterium fidei:||the mystery of faith:|
|qui pro vobis et pro multis effundetur||which shall be shed for you and for many|
|in remissionem peccatorum.||others unto the remission of sins.|
|Haec quotiescumque feceritis,||As often as you shall do these things,|
|in mei memoriam facietis.||you shall do them in memory of me.|