From The Prayers of the Priest: "Mixing the Wine with Water"

This prayer from the Mass is said quietly by the deacon or priest during the Preparation of the Gifts, while he prepares the chalice by pouring in some wine and then adding a few drops of water:

Per huius aquæ et vini mystérium
eius efficiámur divinitátis consórtes,
qui humanitátis nostræ fíeri dignátus est párticeps.

By the mystery of this water and wine              2 Macc. 15:39; John 19:34
may we come to share in the divinity of Christ          Rom. 5:2; 2 Pet. 1:4
who humbled himself to share in our humanity.                       Phil. 2:8

The simple act of pouring water into wine, and the prayer accompanying it, is a synthesis of the whole Mass, of the whole Catholic faith, and of all salvation history.  In order to unearth the theological and doctrinal riches of this easily-overlooked rite, we should first examine the history of this prayer.

Do not be surprised that so much can be written about such a small prayer.  The rite and its prayer are of phenomenal significance, as they represent the totality of redemption, from the Incarnation to the Passion and beyond, to the Resurrection and our eventual sharing in the divine life of God in Heaven.

(Read the whole thing here.)