The angel showed me a river of life-giving water, clear as crystal, issuing from the throne of God and of the Lamb, and flowing down the centre of the city street.

On both banks of the river grew the tree of life, bearing its fruit twelve times a year, one crop for each month. The leaves of that tree are destined for the healing of the nations (Apoc. 22:1-2).

The river here depicted is none other than the torrent of gladness and joy described in one of the psalms as the fast­-flowing river that gladdens the City of God.

Of this  river of another psalm proclaims: They shall be filled with the abundance of your house, O Lord, and you will give them water from the flowing stream of your delights.

The same figurative language was used by Isaiah to console the people of Jerusalem: Thus says the LordI will make peace flow over her like a river; the wealth of the nations shall pour into her like a torrent in full spate.

The river of John’s vision, therefore, represents the Lord. More specifically, we can see in it an image of the Holy Spirit.

It is the Holy Spirit who is the river of peace, the torrent of wealth, the river of gladness, the flowing stream of delight, and the abundance of God’s house.

For he is himself the love that unites bride and Bridegroom in the city of glory and constitutes the entire happiness of all who live there.

That love which is the Holy Spirit is the life of the holy angels and of all saintly souls.

Consequently the river shown to John by the angel is called a river of life-giving water. Because its water imparts light and strength it is said to be clear as crystal.

This is a beautiful comparison. Crystal is a substance which is translucent yet very durable, qualities which we ourselves shall possess in the life of glory.

Our minds will be wholly irradiated with the divine light, and our bodies will gain a crystalline strength through the gift of blessed immortality in that state of eternal happiness where there will be no more dying.

Now we know that in the Gospel of Saint John our Lord speaks of the procession of the Holy Spirit.

When the Advocate comes whom I shall send you, he says, the Spirit of Truth who issues from the Father.

Nor does John neglect that teaching in the Apocalypse. In fact, he affirms it precisely in this very passage, where he tells us that the river issues from the throne of God and of the Lamb.

Rupert of Deutz (c.1075–1129): Commentary on the Apocalypse, 22 (PL 169:1206); from the Monastic Office of Vigils for Friday of the Fifth Week in Eastertide, Year 1.