There is some discussion on the Internet ... again! ... about the phrase 'Mystery of Faith', in the Institution Narrative of the Roman Rite.
I published the following in August last year. In my view, it explains exactly what the phrase is taken to mean, and why it was inserted into the Words of Consecration of the Chalice. So I've sharpened it up a bit and now I reoffer it to you.
A striking characteristic of the Roman Institution Narrative is its determination to gather in whatever can be found in Scripture ... and, by that, we mean the Hebrew Scriptures as well as what we call the New Testament. Too often even Catholics, who should know better, fail to comprehend the Scriptures holistically as a single narrative of Salvation History in which everything relates to everything.
(1) So we say that the Lord took hunc praeclarum calicem ["this excellent chalice"]. This phrase is gathered into the Last Supper Narrative from Psalm 22:5 [The Lord is my shepherd...]. A dreary 'Enlightenment' approach might tediously discuss the date and authorship of the psalm, and would implicitly ignore the Eucharistic reference, obvious to any Catholic or Orthodox, of calix meus quam praeclarus est. But we are Catholics.
(2) The words about the Lord lifting his eyes to heaven ... you will have noticed that these are gathered into the Institution Narrative from the Feedings of the 5,000 and the 4,000 recorded in Scripture, which we recognise as Eucharistic anticipations.
(3) Our Covenant is not only the Covenant [Testament] which lies at the heart of the 'Old Testament'; it is also 'New' (I Corinthians 11:25), and additionally 'Eternal' (Psalm 110:9; Ecclus.17:12; 45: 15: etc.).So "New" and "Eternal" are gathered into the story of the Last Supper.
(4) The most puzzlement is caused by the words Mysterium Fidei. Jungmann rightly dismisses as "poetry, not history" the theory that these were words originally spoken by the Deacon. Baseless myths, however, die hard and after Vatican II it became yet another silly (and illegal) fad to give these words to the Deacon.
I am quite sure that the phrase was gathered into the Consecration of the Chalice from I Timothy 3:9, which talks about the deacons "holding the Mystery of Faith". Since the Deacon was commonly regarded as having a special responsibility for the Chalice (at High Mass he still joins the Priest in offering the Chalice), "holding the Mystery of Faith" was taken to be equivalent to "holding the Chalice".
So "Mystery of Faith" in the Roman rite means the Chalice of the Lord's Blood.
So, just as "this excellent chalice" and "lifting up His eyes to heaven" and "New" Covenant and "Eternal' were gathered into the Institution Narrative from elsewhere in Holy Scripture, so also the Apostle's words about the Deacons "holding" [ekhontas] the "Mystery of Faith" were understood as referring to the Chalice and gathered into the account of the Last Supper.