19 Wherefore, says this Jerusalem, do I praise in unison the Lord, and, as Sion, praise my God? Jerusalem is the same as Sion. For different reasons has it the two names. Jerusalem means “visions of peace;” Sion means “watching.” See whether these words do not sound like sights; that the Gentiles may not think that they have sights and we have none. Sometimes after the theatre or amphitheatre breaks up, when the crowd of lost ones begins to be vomited forth from that den, sometimes, retaining in their minds images of their vain amusements, and feeding their memory with things not only useless but even hurtful, rejoicing in them as if they were sweet, while they are really deadly; they see often, it may be, the servants of God pass by, they recognise them by their garb or headdress, or they know them by sight, and they say to one another, or inwardly, “Wretched people, how much they lose!”
Brethren, let us return their good will (for they do mean it well) with prayers to the Lord. They wish us well; but “he that loves iniquity, hates his own soul.” If he hates his own soul, how shall he love my soul? Yet with a perverse, and empty, and vain good will, if indeed it may be called good will, they grieve that we lose what they love: let us pray that they lose not what we love. Behold of what character that Jerusalem is to be which he exhorts to praise, or rather foresees will praise.
For the praises of that city, when we shall see and love and praise, will not need to be urged on and stirred up by the voice of prophecy; but the Prophets now say this, to drink in as far as while they remain in this flesh they can, the future joys of the blessed, and then giving them forth into our ears, to arouse in us love of that city. Let us burn with longing, let us not be slothful in spirit. “Praise your God, O Sion.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)