What is the purpose (theological) of the mixing of the wine with holy water before the consecration?
As per the prayer said by the priest or deacon while the water is poured into the wine, it symbolizes the hypostatic union–the union of human and divine natures in Christ–and his kenosis, or emptying, in assuming our nature.
“By the mystery of this water and this wine, may we come to share in the divinity of Christ, who humbled himself to share in our humanity.”
And our theosis, i.e. our partaking of the divine nature.
The mixing of wine with water probably also is reminiscent of *John *19:34
But one of the soldiers pierced his side with a spear, and at once there came out blood and water.
This also hearkens to the explanation of the red and white rays streaming from Jesus’ heart in the Divine Mercy image. From St. Faustina’s Diary, 299…
[Jesus said]: The two rays denote Blood and Water. The pale ray stands for the water that makes souls righteous. The red ray stands for the Blood which is the life of souls…
These two rays issued forth from the very depths of My tender mercy when my agonized Heart was opened by a lance on the Cross.
The symbolism is even more striking here in the Byzantine liturgy. As the priest prepares the chalice, he says: “And the soldier thrust his lance into the side of Christ, bringing forth blood” (he pours the wine) “and water.” (he pours the water).