Chrysostom:
But his meaning is like this: If ordinary persons knew when they were going to die, they would surely be striving earnestly at that hour. In order therefore that they may strive, not at that hour only, he does not tell them the hour or day. He wants to keep them on their toes looking for it, that they may be always striving. This is why he made the end of each person’s life so uncertain.
Hilary of Poitiers:
To teach us that our ignorance of the date of his return (which his silence has kept hidden from everyone) is not without its usefulness, Christ warns us to keep all his commandments. We should also be occupied with constant prayer in order to guard against the coming of the thief. For the thief is the devil who seeks to invade our bodily homes with the darts of his thoughts and allurements in order to ruin us while we are sleepy and careless. It is good therefore that we be prepared. Our ignorance of the day of Christ’s return should provoke us to be careful as we eagerly await his coming.