Theophylact of Ohrid

So then, it is necessary to be watchful. For we are like the master of a house. If he does not sleep, the thief cannot steal anything from his possessions; but if he is drowsy, the thief will take everything and leave. Some understand here by the thief the devil, by the house the soul, and by the master of the house man. However, such an understanding does not seem to fit the connection of the discourse. …

Commentary on Luke ↗

Gregory the Dialogist

But to shake off the sloth of our mind, even external losses are brought forward through a comparison, so that through these the soul may be roused to guard itself. For it is said: "Know this, that if the master of the house knew at what hour the thief was coming, he would certainly watch and would not allow his house to be broken into." For while the master of the house is unaware, the thief breaks into the house, because while the spirit sleeps from guarding itself, unforeseen death coming bursts into the dwelling of our flesh, and …

Forty Gospel Homilies, Homily 13 ↗

Bede

But know this, that if the master of the house had known at what hour the thief would come, he would have watched and would not have allowed his house to be broken into. From this preceding analogy, an exhortation is also implied when it is said:

On the Gospel of Luke ↗

Bonaventure

But know this, etc. Here thirdly he subjoins an incitement to watchfulness, which he introduces in this manner, namely by proposing a parabolic example and by concluding with the principal intent. As to the first, therefore, he sets forth the parabolic example when he says: But know this, that if the householder knew at what hour the thief would come, he would certainly watch, namely for the guarding of the house, lest the thief stealthily carry something away. Therefore he says: And he would not suffer his house to be broken into. …

Commentary on Luke, Chapter 12 ↗