8 But as often as water is named alone in the Holy Scriptures, baptism is referred to, as we see intimated in Isaiah: “Remember not,” says he, “the former things, and consider not the things of old. Behold, I will do a new thing, which shall now spring forth; and you shall know it. I will even make a way in the wilderness, and rivers in the dry place, to give drink to my elected people, my people whom I have purchased, that they might show forth my praise.” There God foretold by the prophet, that among the nations, in places which previously had been dry, rivers should afterwards flow plenteously, and should provide water for the elected people of God, that is, for those who were made sons of God by the generation of baptism. Moreover, it is again predicted and foretold before, that the Jews, if they should thirst and seek after Christ, should drink with us, that is, should attain the grace of baptism.
“If they shall thirst,” he says, “He shall lead them through the deserts, shall bring forth water for them out of the rock; the rock shall be cloven, and the water shall flow, and my people shall drink;” which is fulfilled in the Gospel, when Christ, who is the Rock, is cloven by a stroke of the spear in His passion; who also, admonishing what was before announced by the prophet, cries and says, “If any man thirst, let him come and drink. He that believes in me, as the Scripture says, out of his belly shall flow rivers of living water.”
And that it might be more evident that the Lord is speaking there, not of the cup, but of baptism, the Scripture adds, saying, “But this spoke He of the Spirit, which they that believe in Him should receive.” For by baptism the Holy Spirit is received; and thus by those who are baptized, and have attained to the Holy Spirit, is attained the drinking of the Lord's cup. And let it disturb no one, that when the divine Scrip-lure speaks of baptism, it says that we thirst and drink, since the Lord also in the Gospel says, “Blessed are they which do hunger and thirst after righteousness;” because what is received with a greedy and thirsting desire is drunk more fully and plentifully.
As also, in another place, the Lord speaks to the Samaritan woman, saying, “Whosoever drinks of this water shall thirst again; but whosoever drinks of the water that I shall give him, shall not thirst for ever.” By which is also signified the very baptism of saving water, which indeed is once received, and is not again repeated. But the cup of the Lord is always both thirsted for and drunk in the Church.
Source: The Epistles of Cyprian (New Advent)