This section focuses on vigilance. During the period described, people would not act like the people in the days of Noah and ignore the promised judgment. Even if people do not reckon with God, and even if the judgment takes longer than expected, it will come.
42 Watch therefore, for you have not perceived what day your Lord is coming.
43 But know this: If the owner of the house had known in what watch the thief would come, he would have watched, and never suffered his house to be broken up.
44 Therefore, be ready also, for at an hour when you do not think so, the Son of Man comes.
night watches
The call at this time is to be vigilant, like a man who knows that a thief is coming to take something from his house. The homeowner does not know exactly when that thief will come, but he will come and if he is not vigilant, take away his property. So also the Lord will come suddenly, and the unbelievers will be taken away from the earth.
The night is divided into four vigils, we find this several times in Scripture, such as with the shepherds we encounter around the birth of Jesus.
Luke 2
8 And there were in the same country shepherds, which were in the fold of the field, and kept the watch of the night over their flock.
The shepherds kept watch over the flock and apparently they did this on a rotating basis. In Matthew 14:25 there is mention of a fourth watch. And Luke 12:38 speaks of a second and third watch. Here too it is about vigilance (Luke 12:35) and looking for the Lord to come (Luke 12:36). One does not know exactly when to expect the Lord, because one does not know the day and hour when the Lord will come.
Mark and Luke
The parallel passages from the other Gospels also shed light on these verses in Matthew 24.
Mark 13
33 Watch, be alert, and pray. For you do not know when the appointed time will come.
34 Like a man, a traveler, who leaves his house, and gives authority to his servants, and to each man his work, and gives instruction to the doorkeeper, that he may watch.
35 Watch therefore, for you do not know when the master of the house comes, whether late in the evening, or at midnight, or at cockcrow, or early in the morning,
36 lest he come suddenly and find you while you sleep.
37 And what I say to you, I say to all: Watch!
endure and watch to the end
In verse 35 the four watches of the night are mentioned. And here too we find the call to watch. It is contrasted with sleeping. The Lord will come suddenly and believers should be ready and awake and not sleep. For he who endures until the end will be saved and will enter the aeon to come (Matt.24:13).
Luke 21
34 And take heed to yourselves, lest your hearts be heavy with drunkenness, and the cares of the common things of life, and that day come before you unexpectedly.
35 For he shall fall like a snare upon all who dwell on the face of the whole earth.
36 And be watchful, and make supplication at every opportunity, that you may be able to escape all these things that are to come, and to stand before the Son of man.
deception
If, as in the days of Noah, one simply continues to live without taking into account the coming of the Messiah and His Kingdom, one will be taken by surprise. Or if one is not sober and sensible, but lives in intoxication and drunkenness. That can be literal, but one can also be intoxicated by certain pursuits (Luke 21:34; Matt. 24:38), false claims of men (1 Thess. 5:3) and religion.
For then there will be many false prophets who will deceive many (Matt.24:11). They claim to speak for God and pretend to be ‘ministers of righteousness’, but they are servants of Satan (2 Cor. 11:13-15). Therefore, one would be vigilant and rely on the word of God.