Theosis

Deus, qui humanæ substantiæ dignitatem mirabiliter condidisti, et mirabilius reformasti: da nobis* ejus divinitatis esse consortes, qui humanitatis nostræ fieri dignatus est particeps, Jesus Christus Filius tuus Dominus noster: qui tecum vivit et regnat in unitate Spiritus Sancti, Deus, per omnia sæcula sæculorum. Amen. 

(O God, Who wonderfully didst create the dignity of human nature, and still more wonderfully didst restore it: grant to us* that we may be sharers in His divinity, Who didst deign to share in our humanity, Jesus Christ thy Son our Lord: Who with thee liveth and reigneth in the unity of the Holy Ghost, God, world without end. Amen.)

This Collect, with per hujus vini et aquæ mysterium ("by the mystery of this water and wine") inserted after the asterisk, is used daily in the Extraordinary Form of the Roman Mass; the original text, however, was more or less as given above, prior to its adoption as part of the offertory rite; and in the modern Ordinary Form, this Collect, slightly altered (with et also before mirabiliter, and quæsumus, before nobis), is used at the Day Mass of Christmas, while the central portion of it is still used daily at the mixing of the chalice, with esse replaced by efficiamur – Per hujus aquæ et vini mysterium ejus efficiamur divinitatis consortes, qui humanitatis nostræ fieri dignatus est particeps.

It is sometimes claimed that the Western Church focusses less on deification, theosis, than the Eastern: but seeing as the words of II Peter i, 4 about becoming "partakers of the divine nature" (divinæ consortes naturæ) are repeated in this Collect, whether at length or in abbreviated form, every day by every priest at the altar, whether in the older or newer forms of Mass (unless, say, a Dominican priest is offering the Sacrifice according to the proper Use of his Order), it can hardly be asserted that this doctrine is not prayed about very often indeed...

Another striking prayer, the Secret of the Masses of both the 4th Sunday after Easter and the 19th Sunday after Pentecost (EF; used a number of times in Eastertide in the OF), states that God makes us partakers of the one supreme Godhead by the rightly revered exchange of the Eucharistic Sacrifice – Deus, qui nos per hujus sacrificii veneranda commercia unius summæ divinitatis participes effecisti... Examples of this sort might be multiplied.

Especially at this Christmas season, we ought thankfully contemplate the unimaginable blessing whereby our good God did not merely (!) create us, but in a manner beyond all hope redeemed us, His Son becoming what He was not, while remaining what it was before, that, having vouchsafed to take upon himself our human nature, we might be made partakers of the one supreme divinity.  As one of the Fathers said (and the rest repeated), God became man, that men might be made gods – what the evil serpent lyingly promised could be achieved through sinful disobedience, God's Incarnate Son made truly possible to all engrafted into Him by graced obedience.

Just as power went forth from Him when the Hæmorrhissa dared to touch the fringe of His garment, so, too, the reception of every Sacrament in love and fear and faith is in truth the physical touch of Christ's Sacred Humanity, the Instrument of the Divinity, imparting life and grace, healing and salvation to us, and no mere moral connexion, no bare memorial. In the daring words of Suarez, Our Lady at the Annunciation "touched the hem of the Divinity"; we in our humility must dare to do likewise.  Thus we will be supernaturally uplifted beyond our nature, elevated, divinized, made capax Dei, able to enter Heaven and abide in God: and this is theosis.