13 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 is 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.
Source: The Epistles of Cyprian (New Advent)