St. Thomas Aquinas wrote:
"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.’
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