Isaiah 43:20

But He, although He took on Him our flesh, although He was made man that He might redeem man, and recal him from death, still, as being God, came upon earth in an extraordinary way, that as He had said, 'Behold I make all things new,' so also He might be born of an immaculate virgin, and be believed to be, as it is written, 'God with us.' But from their perverse ways they are induced to say 'She was a virgin when she conceived, but not a virgin when she brought forth.'

For of this number I find figurative hints up and down the Creator's dispensation in the twelve streams of Elim. … Now the same number of apostles was thus portended, as if they were to be fountains and rivers that should water the Gentile world, which was formerly dry and destitute of knowledge, as he says by Isaiah, "I will put streams in the unwatered ground."

What I am going to do, he says, merits still more admiration than the things done [in the past]. He adds what will be: "And I will make a way in the wilderness and rivers in the dry land." Some have thought that these declarations …

"Because I have given water in the wilderness and rivers in the waterless land to assuage the thirst of my elect race; the people I have set aside for myself proclaim my virtues." The race that [Jesus] put on according to his human nature is …

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.…" There God foretold by the prophet that among the nations, in places that 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 children 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 anyone thirst, let him come and drink. He who 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.