We came across this interesting piece about wine on a Vatican web site (ZENIT - The World Seen From Rome).

ROME, JUNE 29, 2004 (Zenit.org).- Answered by Father Edward McNamara, professor of liturgy at the Regina Apostolorum Pontifical University.

Q: I would want to know the reason why the priest pours water into wine during the preparation of the gifts. -- J.B., Bo, Sierra Leone

A: The brief rite of pouring water into the wine used for consecration is very ancient. Indeed, it is believed that Our Lord himself used wine tempered with water at the Last Supper as this was the common practice among the Jews and in Mediterranean culture in general.

Some form of this is found in practically every rite of the Church both Western and Eastern, except for a group of Armenian Monophysites.

Although the water is not essential for the validity of the sacrament, the Church holds it in great importance and it must never be omitted. The Council of Trent even went so far as to excommunicate whoever denied the need for this mixture (see Canon 9, Session XXII).

Historically, St. Justin Martyr already mentions this practice in his Apology around the year 150. About a century later St. Cyprian wrote on this theme in an epistle against a splinter group that used only water in their celebrations, and this has become the accepted interpretation:

"For because Christ bore us all, in that He also bore our sins, we see that in the water is understood the people, but in the wine is showed the blood of Christ. But when the water is mingled in the cup with wine, the people [are] made one with Christ, and the assembly of believers is associated and conjoined with Him on whom it believes; which association and conjunction of water and wine is so mingled in the Lord's cup, that that mixture cannot any more be separated.

"Whence, moreover, nothing can separate the Church -- that is, the people established in the Church, faithfully and firmly persevering in that which they have believed -- from Christ, in such a way as to prevent their undivided love from always abiding and adhering. Thus, therefore, in consecrating the cup of the Lord, water alone cannot be offered, even as wine alone cannot be offered. For if any one offer wine only, the blood of Christ is dissociated from us; but if the water be alone, the people are dissociated from Christ; but when both are mingled, and are joined with one another by a close union, there is completed a spiritual and heavenly sacrament.

"Thus the cup of the Lord is not indeed water alone, nor wine alone, unless each be mingled with the other; just as, on the other hand, the body of the Lord cannot be flour alone or water alone, unless both should be united and joined together and compacted in the mass of one bread; in which very sacrament our people are shown to be made one, so that in like manner as many grains, collected, and ground, and mixed together into one mass, make one bread; so in Christ, who is the heavenly bread, we may know that there is one body, with which our number is joined and united" ("On the Sacrament of the Cup of the Lord," No 13).

Another important symbolic explanation for this rite is given in St. Thomas Aquinas, in his Summa Theologiae, III pars q 74, 6-8:

"Water ought to be mingled with the wine which is offered in this sacrament.

"First of all, on account of its institution: for it is believed with probability that our Lord instituted this sacrament in wine tempered with water according to the custom of that country: hence it is written (Proverbs 9:5): 'Drink the wine which I have mixed for you.'

"Secondly, because it harmonizes with the representation of our Lord's Passion: hence Pope Alexander I says (Ep. 1 ad omnes orth.): 'In the Lord's chalice neither wine only nor water only ought to be offered, but both mixed because we read that both flowed from His side in the Passion.'

"Thirdly, because this is adapted for signifying the effect of this sacrament, since as Pope Julius says (Concil. Bracarens iii, Can. 1): 'We see that the people are signified by the water, but Christ's blood by the wine. Therefore when water is mixed with the wine in the chalice, the people [are] made one with Christ.'

"Fourthly, because this is appropriate to the fourth effect of this sacrament, which is the entering into everlasting life: hence Ambrose says (De Sacram. v): 'The water flows into the chalice, and springs forth unto everlasting life.'"

These different explanations form the basis for the Church's understanding of the importance of this rite. This understanding is at the root of the sentiment expressed by the prayer which the priest recites in a low voice as he pours the water into the chalice:

"By the mystery of this water and wine may we come to share in the divinity of Christ, who humbled himself to share in our humanity."