He only comes as a thief upon those who are not watching. Jesus told the church at Sardis, “Be watchful, . . . if you will not watch, I will come upon you as a thief, and you will not know what hour I will come upon you” (Rev. 3:3b+). See commentary on Revelation 3:3.
Those who are in Christ are to be constantly on the lookout for His arrival, not that of Antichrist:
Watch therefore, for you do not know what hour your Lord is coming. But know this, that if the master of the house had known what hour the thief would come, he would have watched and not allowed his house to be broken into. Therefore, you also be ready, for the Son of Man is coming at an hour you do not expect. (Mtt. 24:42-44)
Take heed, watch and pray; for you do not know when the time is. It is like a man going to a far country, who left his house and gave authority to his servants, and to each his work, and commanded the doorkeeper to watch. Watch therefore, for you do not know when the master of the house is coming-in the evening, at midnight, at the crowing of the rooster, or in the morning- lest, coming suddenly, he find you sleeping. And what I say to you, I say to all: “Watch!” (Mark 13:33-37)
Let your waist be girded and your lamps burning; and you yourselves be like men who wait for their master, when he will return from the wedding, that when he comes and knocks they may open to him immediately. Blessed are those servants whom the master, when he comes, will find watching. Assuredly, I say to you that he will gird himself and have them sit down to eat, and will come and serve them. And if he should come in the second watch, or come in the third watch, and find them so, blessed are those servants. But know this, that if the master of the house had known what hour the thief would come, he would have watched and not allowed his house to be broken into. Therefore you also be ready, for the Son of Man is coming at an hour you do not expect. (Luke 12:35-40)
Jesus indicated that a day was coming which would arrive unexpectedly as a snare to the earth dwellers, but by vigilance and prayer, the watchful believer could escape the things that were coming to pass:
But take heed to yourselves, lest your hearts be weighed down with carousing, drunkenness, and cares of this life, and that Day come on you unexpectedly. For it will come as a snare on all those who dwell on the face of the whole earth. Watch therefore, and pray always that you may be counted worthy to escape all these things that will come to pass, and to stand before the Son of Man. (Luke 21:34-36)
The day which He spoke of was The Day of the Lord which professing but unbelieving “Christians,” who miss the Rapture, will endure along with those dwelling upon the earth. For them, the beginning of the end comes as a surprise since they are not expecting it. It arrives, as a thief