The reason that people believe that Paul is saying that the man of sin must come before the church is gathered, is because they misread the scripture, which I am going to explain as I have in previous posts. Below is the scripture in question:
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Now concerning the coming of our Lord Jesus Christ and our being gathered together to Him, we ask you, brothers,
2not to be easily disconcerted or alarmed by any spirit or message or letter seeming to be from us, alleging that
the Day of the Lord has already come.
3Let no one deceive you in any way, for
that day will not come until the rebellion occurs and the man of lawlessness—the son of destruction—is revealed.
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If you will notice in the scripture above, Paul begins with "the coming of our Lord and our being gathered to Him." Then in verse 2, he makes reference to the Day of the Lord, which though close in proximity, are two different events.
Our being gathered to the Lord, is when He appears and calls up His church, with the resurrection of the dead believers taking place first and then those believers who are still alive being changed and caught up with them. In Paul's letter to Titus, he calls our gathering the blessed hope of which Paul said to comfort one another with those words.
In opposition, "The Day of the Lord" is the time of God's wrath. People miss this change in the scripture from "the coming of our Lord and our being gathered to Him" vs. "The Day of the Lord," interpreting them as being the same event, which they are not. Because of their error of not recognizing Paul's change from 'the Lord's coming and our being gathered to Him' to 'the Day of the Lord' they interpret it as in the following paraphrase:
Now concerning the coming of our Lord Jesus Christ and our being gathered together to Him, we ask you, brothers,
2not to be easily disconcerted or alarmed by any spirit or message or letter seeming to be from us,
alleging that the coming of our Lord and our being gathered to Him has already come. 3Let no one deceive you in any way, for that day will not come until the rebellion occurs and the man of lawlessness—the son of destruction—is revealed.
Do you see the error? Paul referred to 'the Day of the Lord" and not 'the coming of our Lord and our being gathered to Him.'
The scripture is saying that "the Day of the Lord" will not come until the apostasy occurs and the man of lawlessness is revealed, not our being gathered.
* The coming of our Lord and our being gathered to Him = the catching up of the church (rapture)
* The Day of the Lord = The time period of God's wrath
The coming of our Lord and our being gathered to Him takes place first, with the Day of the Lord beginning thereafter.
I studied the heck out of this scripture, inside out and upside down, because I wanted to understand it, because I knew that since Jesus already suffered God's wrath on behalf of every believer, satisfying it completely, that believers could not and would not go through the time of His wrath. However, the answer becomes very clear once you recognize Paul's change from "the coming of the Lord and our being gathered to Him" vs. "The Day of the Lord," also referred to in verse 3 as 'that day.'
May the Lord open your eyes to this truth, as well as to everyone else who is reading this.