Luke 12:39

And this know
The Ethiopic version reads, "this only know"; only take notice of this one thing, and it may be of some use to direct you in your conduct how to behave during the absence of your Lord, until the time he shall come again:

that if the good man of the house had known what hour the thief
would come;
that is, if the owner, or master of the house, whose the goods in the house are, could by any means know what time of the night the thief would come to break into his house, in order to plunder it, and carry off his goods:

he would have watched;
either he himself in person, or he would have set a watch about his house, or in it:

and not have suffered his house to be broken through;
either the door to be broken up, or the wall to be dug through, but by a guard about it, or within it, would have prevented such a design. And so in like manner, could it be known in what time Christ would come, either to the destruction of Jerusalem, or at death, or to judgment, every thoughtful, prudent man that should know it, would be upon his guard, that he might not be surprised with it; and though the precise time could not be known, yet inasmuch as the thing itself is certain, it became all the servants of Christ to be watching for it; (See Gill on Matthew 24:43).