Hello PaulNew,
The answer to your question is found right in v.10 & 13 in the context of the parable of the ten virgins:
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“At that time the kingdom of heaven will be like ten virgins who took their lamps and went out to meet the bridegroom.
2Five of them were foolish, and five were wise.
3The foolish ones took their lamps but did not take along any extra oil.
4But the wise ones took oil in flasks along with their lamps.
5When the bridegroom was delayed, they all became drowsy and fell asleep.
6At midnight the cry rang out: ‘Here is the bridegroom! Come out to meet him!’
7Then all the virgins woke up and trimmed their lamps.
8The foolish ones said to the wise, ‘Give us some of your oil; our lamps are going out.’
9‘No,’ said the wise ones, ‘or there may not be enough for both us and you. Instead, go to those who sell oil and buy some for yourselves.’
10But while they were on their way to buy it, the bridegroom arrived.
Those who were ready went in with him to the wedding banquet, and the door was shut.
11Later the other virgins arrived and said, ‘Lord, lord, open the door for us!’
12But he replied, ‘Truly I tell you, I do not know you.’
13Therefore keep watch, because you do not know the day or the hour.
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Those who were ready
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Therefore keep watch
The five foolish virgins took their lamps but no extra oil. The lamps give light. The fact that their lamps were going out and not having any oil, infers that they will have turned from their faith (lamps going out) and because of that will not be watching and ready because they are caught up in the cares of this world and back living according to the sinful nature. In that state they are no longer watching and ready for the Lord's appearing to gather His church. Jesus warns believers about this in Luke 21
"“Be careful, or your hearts will be weighed down with carousing, drunkenness and the anxieties of life, and that day will close on you suddenly like a trap. For it will come on all those who live on the face of the whole earth.
Be always on the watch, and pray that you may be able to escape all that is about to happen, and that you may be able to stand before the Son of Man.”
The reference to their hearts being weighed down with carousing, drunkenness and the cares of this life, is referring to believers falling away from faith and going back to willfully living according to the sinful nature. As I said, in this state a believer will not be watching nor will they be ready when that voice that sounds like a trumpet calls us up to meet Him in the air.
Sadly, there will be many that will be in this state when the Lord comes to gather the church and the Day of the Lord will close on them like a trap, meaning that they will remain on the earth during the time of God's wrath via the seals, trumpets and bowl judgments and the beasts kingdom. Many of them will probably become those great tribulation saints. In any case, when the church is removed they will understand what has happened and will know the time period that they are in and what they are going to have to go through for their faith in Jesus, which will be the worst time in the history of the world, from the beginning, until now and never to be equaled again.
I hope this is beneficial