It is because of this verse here
Let no one deceive you by any means; for that Day will not come unless the falling away comes first, and the man of sin is revealed, the son of perdition, who opposes and exalts himself above all that is called God or that is worshiped, so that he sits as God in the temple of God, showing himself that he is God. Do you not remember that when I was still with you I told you these things? And now you know what is restraining, that he may be revealed in his own time. For the mystery of lawlessness is already at work; only He who now restrains will do so until He is taken out of the way. And then the lawless one will be revealed, whom the Lord will consume with the breath of His mouth and destroy with the brightness of His coming. (2 Thess. 2:3-8)
I used to believe in a pretrib rapture but if it was true then what does this verse mean then?
Mark 13 tells you the rapture is after the tribulation.
Christ second coming means our salvation and the wicked world's destruction as Hebrews 9:28 says
Christ's second coming ends the tribulation period which comes the church, and the tribulation is even today, just not as extreme as it can be.
21 “Then if anyone says to you, ‘Look, here
is the Christ!’ or, ‘Look,
He is there!’ do not believe it. 22 For false christs and false prophets will rise and show signs and wonders to deceive, if possible, even the [
g]elect. 23 But take heed; see, I have told you all things beforehand.
The Coming of the Son of Man
24
“But in those days, after that tribulation, the sun will be darkened, and the moon will not give its light; 25 the stars of heaven will fall, and the powers in the heavens will be shaken. 26 Then they will see the Son of Man coming in the clouds with great power and glory. 27 And then He will send His angels, and gather together His [h]elect from the four winds, from the farthest part of earth to the farthest part of heaven.
The Parable of the Fig Tree
28 “Now learn this parable from the fig tree: When its branch has already become tender, and puts forth leaves, you know that summer is near. 29 So you also, when you see these things happening, know that [
i]it is near—at the doors! 30 Assuredly, I say to you, this generation will by no means pass away till all these things take place. 31 Heaven and earth will pass away, but My words will by no means pass away.
No One Knows the Day or Hour
32 “But of that day and hour no one knows, not even the angels in heaven, nor the Son, but only the Father. 33 Take heed, watch and pray; for you do not know when the time is. 34
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. 35 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— 36 lest, coming suddenly, he find you sleeping. 37 And what I say to you, I say to all: Watch!”
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Another example of post tribulation rapture. Rapture is simply a word that speaks of us being gathered to Christ, both living and departed saints of God.
2 Thessalonians 1
God’s Final Judgment and Glory
3 We are bound to thank God always for you, brethren, as it is fitting, because your faith grows exceedingly, and the love of every one of you all abounds toward each other, 4 so that we ourselves boast of you among the churches of God for your patience and faith in all your persecutions and [
a]tribulations that you endure, 5
which is manifest[
b] evidence of the righteous judgment of God, that you may be counted worthy of the kingdom of God, for which you also suffer;
6 since
it is a righteous thing with God to repay with [
c]tribulation those who trouble you, 7 and to
give you who are troubled rest with us when the Lord Jesus is revealed from heaven with His mighty angels, 8 in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ. 9 These shall be punished with everlasting destruction from the presence of the Lord and from the glory of His power, 10 when He comes, in that Day, to be glorified in His saints and to be admired among all those who [
d]believe, because our testimony among you was believed.