The water that I shall give them will become in them 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 person 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 his action, by the will of God and in the name of Christ, are both many and marvellous.
The Spirit makes some teachers of divine truth, inspires others to prophesy, gives others the power of casting out devils, enables others to interpret holy Scripture.
The Spirit strengthens the self-control of some, shows others how to help the poor, teaches others to fast and lead a life of asceticism, makes others oblivious to the needs of the body, trains others for martyrdom.
His action is different in different people, but the Spirit himself is always the same. In each man, Scripture says, the Spirit reveals his presence in a particular way for the common good.
The coming of the Spirit is gentle, his presence fragrant, his weight very light. Rays of light and knowledge stream before him as he approaches.
The Spirit comes with the tenderness of a true friend and protector to save, to heal, to teach, to counsel, to strengthen, to console.
The Spirit comes to enlighten the mind first of the one who receives him, and then through that person the minds of others as well.
As light strikes the eyes of those who come out of darkness into the sunshine and enables them to see clearly things they could not discern before, so does light flood the souls of those counted worthy of receiving the Holy Spirit and enables them to see things beyond the range of human vision of which they had previously been ignorant.
Cyril of Jerusalem (c. 313-386): Catechesis 16, 11-12, 16; from the Monastic Office of Vigils, Easter Friday, Year 2.