From a catechetical instruction by Saint Cyril of Jerusalem, bishop
The water that I shall give him will become in him a fountain of living water, welling up into eternal life. This is a new kind of water, a living, leaping water, welling up for those who are worthy. But why did Christ call the grace of the Spirit water? Because all things are dependent on water; plants and animals have their origin in water. Water comes down from heaven as rain, and although it is always the same in itself, it produces many different effects, one in the palm tree, another in the vine, and so on throughout the whole of creation. It does not come down, now as one thing, now as another, but while remaining essentially the same, it adapts itself to the needs of every creature that receives it.
In the same way the Holy Spirit, whose nature is always the same, simple and indivisible, apportions grace to each man as he wills. Like a dry tree which puts forth shoots when watered, the soul bears the fruit of holiness when repentance has made it worthy of receiving the Holy Spirit. Although the Spirit never changes, the effects of this action, by the will of God and in the name of Christ, are both many and marvelous. […]
As light strikes the eyes of a man who comes out of darkness into the sunshine and enables him to see clearly things he could not discern before, so light floods the soul of the man counted worthy of receiving the Holy Spirit and enables him to see things beyond the range of human vision, things hitherto undreamed of.
Orthodox monastery in Bucovina, 14th-15th century, Romania.