20 He says, “He has made strong the bars of your gates”. The making bars strong is not for open gates, but shut ones, wherefore most manuscripts read, “He has made strong the bolts of your gates.” Observe, beloved. He bids Jerusalem when closed in to praise the Lord. We praise in unison now, we praise now; but it is amid offenses. Many where we wish not, enter in: many though we wish it not, go out: therefore offenses are frequent. “And because iniquity has abounded,” says the Truth, “the love of many waxes cold:” because men come in whom we cannot discern, because men go out whom we cannot retain. Wherefore is this? Because not yet is there perfection, not yet is there the bliss that shall be. Wherefore is this? Because as yet it is the threshing-floor, not yet the garner. What therefore will be then, save no fear that anything of this kind will happen? He said not only, He has set, but, “He has made strong the bars of your gates.” Let none go out, let none come in. Let none go out, we rejoice: let none come in, we fear. Nay, fear not this: when you have entered it will be said: only be thou in the number of virgins, who carried their oil with them....
21. “He has blessed your children within you.” Who? He “who has set peace as your borders.” How ye all exult! Love peace, my brethren. Greatly are we delighted, when the love of peace cries from your hearts. How greatly does it delight you! I had said nothing: I had explained nothing: I but read the verse, and you shouted. What was it that shouted in you? The love of peace....children of the kingdom, O citizens of Jerusalem, in Jerusalem is the vision of peace: and all who love peace are blest in her, and they enter in, when the doors are being shut, and the bars made strong. This, which when but named ye so love and esteem, this follow after, this long for: this love in your home, in your business, in your wives, in your sons, in your slaves, in your friends, in your enemies....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)