Nehemiah 7:1

But after the wall was built, etc. And in a spiritual sense, wherever the wall of the Church may be built, gathering new peoples to the faith, or correcting those who have strayed, immediately the gates of regular discipline should be set up, so that the ancient enemy, who goes about like a roaring lion (I Peter V), may not anywhere break into the fold of the faithful. The gatekeepers, singers, and Levites must be reviewed, who guard the same gates; it is clear that the role of these pertains to all holy teachers. For gatekeepers are those who have received the keys of the kingdom of heaven, so that they may admit the worthy and humble, but keep out the proud and impure from entering the heavenly city, saying: 'You have no part or share in this matter; for your heart is not right before God' (Acts VIII). Singers are those who proclaim with a devout voice the sweetness of the same heavenly homeland to their listeners. Levites are those who always watch over the service of divine worship. Nehemiah commanded that the gates of Jerusalem not be opened until the heat of the sun, that is, throughout the entire night, so that neither the enemy covered by darkness might break in, nor any incautious citizen going out be captured and perish by the enemy. Which also the guardians of souls ought to diligently do throughout the whole night of this age, lest, neglecting the observance of a pious conversation, the devil either enter to disturb the company of the faithful or seize and destroy any of their number among the faithful. However, when the Sun of righteousness appears and the light of future blessedness shines forth, there will no longer be need for the cloisters of continence; for the adversaries will no longer be given the power to attack or tempt the faithful, since they and their prince will be condemned to eternal retribution. Wherefore John in his Apocalypse says of the future glory of the same holy city: 'And its gates will never be shut by day, for there will be no night there' (Rev. XXI)."