Let us drink waters out of our own cisterns and out of our own springing wells. We drink of living water springing up into everlasting life. But this is what the Saviour said of the Spirit, which those who believe on him should receive. For observe what he says: “He who believes on me – not simply this, but – as the Scripture has said – here he sends you back to the Old Testament – out of his belly shall flow rivers of living water.” These are not rivers perceived by sense that merely water the earth with its thorns and trees. But these are rivers that bring souls to the light. And in another place he says: “But the water that I shall give him shall be in him a well of living water springing up into everlasting life” – a new kind of water living and springing up, springing up to those who are worthy.

And why did He call the grace of the Spirit water? Because by water all things subsist; because water brings forth grass and living things; because the water of the rain showers down from heaven; because it comes down in one form but works in many forms. For one fountain waters the whole of paradise, and one and the same rain comes down on all the world, yet it becomes white in the lily, and red in the rose, and purple in violets and hyacinths, and different and varied in each. So it is one in the palm tree, and another in the vine, and all in all things; and yet it is one in nature, not diverse from itself. For the rain does not change itself and come down first as as one thing, then as another, but adapting itself to the constitution of each thing that receives it, it becomes to each what is suitable. And so the Holy Spirit also, being one, and of one nature and indivisible, distributes to each His grace as He wills. And as the dry tree, after being nourished with water, puts forth shoots, so also the soul in sin, when it has been through repentance made worthy of the Holy Spirit, brings forth clusters of righteousness. And though He is one in nature, yet many are the virtues He inculcates by the will of God and in the name of Christ.

Saint Cyril of Jerusalem, Catechetical Lectures, 16.11-12, in Joel C. Elowsky (ed). John 1-10 (Ancient Christian Commentary on Scripture), 266.