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....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)