"When he shall come to be glorified in..." yes, is His Second Coming to the earth; but the part of the sentence saying "...[admired in all them having believed] BECAUSE 'the testimony of us to you' was believed in that day" refers to that future, specific, limited time-period leading up to His Return to the earth Rev19, i.e. DURING the trib yrs.
What I don't get is why you work hard to squeeze scriptures into a pre-trib rubric to begin with. Where is the Biblical evidence for a pre-trib rapture in the first place to motivate you to try to make it fit? I can understand if you had a couple of actual scriptures that you needed to see how they fit together. But trying to fit scripture to this theory that is so external to scripture... I don't get that.
Do you have scripture that says that the 'day' is a seven year time period and that the rapture occurs first? If the 'day' refers to the seven year time period, then you have to scramble to make it fit with chapter 2.
1 Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him,
2 That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.
3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
....
8 And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming:
9 Even him, whose coming is after the working of Satan with all power and signs and lying wonders,
The coming of our Lord doesn't occur until the man of sin is revealed. And 'that wicked' is destroyed at the brightness of Jesus coming.
There is no evidence that Jesus comes back two more times. Pre-trib turning this into two comings of Jesus, or one long year coming of Jesus, is just unwarranted.
1. No where do we see scripture lay out a timeline or scenario with pretrib in it.
2. Paul writes about the coming of Christ as if it were one event.
3. The dead are raised at Jesus coming, the rapture occurs at Jesus coming, and 'that wicked' is destroyed at the brightness of His coming.
Pre-tribbers assume pre-trib and then try to interpret passages around a preconceived pre-trib scenario, which they so often treat as axiomatic truth as if it were scripture, without giving scripture to support it. There is a difference between offering Biblical evidence for pre-trib and presenting some way of interpreting a passage to try to make it fit with a pre-trib scenario.
Attempts at evidence for pre-trib are loose or inferential type arguments. There is a verse that says we are not appointed unto wrath, but to obtain salvation through our Lord Jesus Christ. Because there is a reference to the bowls of wrath in Revelation... pretribbers argue that 'not appointed unto wrath' must mean a rapture before the tribulation. But that would mean that the tribulational saints are 'appointed unto wrath' by that logic. But the saints in Revelation are presented in a positive light, as saints that endure. They overcome the Devil by the blood of the Lamb and the word of their testimony.
Then 'come up hither'... spoken to John... is supposed to be hyperallegorically to mean the church is going to be raptured.... according to those who are supposed to hold to literal interpretation.
But where is the passage that shows a rapture occurring before the tribulation, or even teaches it? To justify treating the coming of Jesus as two events, you need to have some actual evidence.... not the ability to try to reinterpret passages to fit with an extra-Biblical theory.
IOW, "[to be admired] in all those having believed, because 'the testimony of us to you' WAS BELIEVED 'IN THAT DAY'" refers not to the Thessalonians' own "belief" (past and present, in their own time), but to that future time-period preceding His "return" to the earth, but following "our Rapture" (THOSE people--the ones who WILL be coming to faith in the Trib yrs)...
He already clarifies that his readers would receive rest/relief at the Lord's coming. He doesn't break believers down into different time periods. Jesus comes to be admired by them that believe, and the Thessalonians had believed.