13 The Holy City is not the Church of this country only, but of the whole world as well: not that of this age only, but from Abel himself down to those who shall to the end be born and believe in Christ, the whole assembly of the Saints, belonging to one city; which city is Christ's body, of which Christ is the Head. There, too, dwell the Angels, who are our fellow citizens: we toil, because we are as yet pilgrims: while they within that city are awaiting our arrival.
Letters have reached us too from that city, apart from which we are wandering: those letters are the Scriptures, which exhort us to live well. Why do I speak of letters only? The King himself descended, and became a path to us in our wanderings: that walking in Him, we may neither stray, nor faint nor fall among robbers, nor be caught in the snares that are set near our path. This character, then, we recognise in the whole Person of Christ, together with the Church....He Himself is our Head, He is God, co-equal with the Father, the Word of God, by whom all things were made: but God to create, Man to renew; God to make, Man to restore.
Looking upon Him, then, let us hear the Psalm. Listen, beloved. This is the teaching and doctrine of this school, which may enable you to understand, not this Psalm only, but many, if you keep in mind this rule. Sometimes a Psalm, and all prophecy as well, in speaking of Christ, praises the Head alone, and sometimes from the Head goes to the Body, that is, the Church, and without apparently changing the Person spoken of: because the Head is not separate from the Body, and both are spoken of as one...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)