It’s from the Tridentine/Extraordinary form of the mass:
Here’s what he was trying to pray but only makes it up to the word I’ve bold-typed: “mysterium”:
Deus, qui humanae substantiae dignitatem mirabiliter condidisti, et mirabilius reformasti: da nobis per huius aquae et vini
mysterium, eius divinitatis esse consortes, qui humanitatis nostrae fieri dignatus est particeps, Iesus Christus Filius tuus Dominus noster: Qui tecum vivit et regnat in unitate Spiritus Sancti Deus: per omnia saecula saeculorum. Amen.
Translation (again, he only makes it up to the word “Mysterium,” here translated/bold-typed as “mystery”:
O God, Who, in creating human nature, didst wonderfully dignify it, and still more wonderfully restore it, grant that, by the
Mystery of this water and wine, we may be made partakers of His divine nature, who vouchsafed to be made partaker of our human nature, even Jesus Christ our Lord, Thy Son, Who with Thee, liveth and reigneth in the unity of the Holy Ghost, God: world without end. Amen.
I just cut and pasted the Latin and English text from
dailycatholic.org/holymas3.htm
but if you have any specific questions about the grammar I could help
