Gregory the Great — as excerpted in the Catena Aurea on Mark 13:32-37

“(Hom. in Evan. 9) For the earth is properly the place for the flesh, which was as it were carried away to a far country, when it was placed by our Redeemer in the heavens. And he gave his servants power over every work, when, by giving to His faithful ones the grace of the Holy Ghost, He gave them the power of serving every good work. He has also ordered the porter to watch, because He commanded the order of pastors to have a care over the Church committed to them. Not only, however, those of us who rule over Churches, but all are required to watch the doors of their hearts, lest the evil suggestions of the devil enter into them, and lest our Lord find us sleeping. Wherefore concluding this parable He adds, Watch ye therefore: for ye know not when the master of the house cometh, at even, or at midnight, or at cockcrow, or in the morning: lest coming suddenly he find you sleeping.”