The thought implied in this verse is that our Lord's Return would be thief-like in its manner (1 Thes. 5:1-6; 2 Pet. 3:10; Rev. 16:15). When a thief comes to break into a house, does he carry a bright light and blow a trumpet, shouting to the people, "Hello, you people! Wake up! I am coming to rob your houses"? Of course we know he does no such things. Neither, therefore, does our Lord when He returns as a thief in the night. The thief in the night comes silently, stealthily, hiding in the dark, walking noiselessly with padded shoes or in stocking feet, picking locks and opening doors or windows with quiet deftness, thus concealing his presence from his unsuspecting victims.
Our Lord in the first part of His Second Advent comes in a similar manner, unknown to the world. But as a thief's presence in the house that is being robbed may by certain signs be made known to his accomplices who may be awaiting him in or at that particular house, so the Lord promised to make known, after His Return, to His faithful watching people, the fact of His Return by certain signs and proofs (Matt. 24:3, 30-33).
Note how 1 Thes. 5:1-6 tells us that the world would not be aware of our Lord's Return, because of its thief-like manner, but that His waking and watchful people would be aware of it. Of course, if the world would see His Return with their physical eyes, they would be aware of it. So their not being aware of it proves that they would not see Him physically in His Second Advent, which therefore must be invisible to physical sight.
Our Lord in the first part of His Second Advent comes in a similar manner, unknown to the world. But as a thief's presence in the house that is being robbed may by certain signs be made known to his accomplices who may be awaiting him in or at that particular house, so the Lord promised to make known, after His Return, to His faithful watching people, the fact of His Return by certain signs and proofs (Matt. 24:3, 30-33).
Note how 1 Thes. 5:1-6 tells us that the world would not be aware of our Lord's Return, because of its thief-like manner, but that His waking and watchful people would be aware of it. Of course, if the world would see His Return with their physical eyes, they would be aware of it. So their not being aware of it proves that they would not see Him physically in His Second Advent, which therefore must be invisible to physical sight.