The word used for "temple" here is not used by Paul to refer to a literal temple whenever he uses it elsewhere.
I think it is possible for those looking for a literal fulfillment of "he as God sitteth in the temple of God" could miss the emergence of a religious leader from within the Christian world.
It could mean the jews rebuild what is commonly thought of as the temple of God, which it is not a real temple of God, look who sits there the devil!
Or it could mean this false christ, devilish demonic religious leader of the last day near the very end could sit in the minds of church people who think he is Christ returned.
29 So you also, when you see these things happening, know that [
i]it is near—at the doors!
Paul also taught the church to not be deceived that the Day of Christ had come, as that figure in 2 Thessalonians 2 has to first show up on the scene, and perform false signs and wonders to deceive all people, except for the elect. And after he appears, and is worshipped as god, only then does Christ return, so if you ever see such a thing, know that Christ's return is at the doors, like about to happen.
Jesus warned of this, and Paul builds on it in 2 Thessalonians 2, about the great apostasy of the end time.
Mark 13
20 And unless the Lord had shortened those days, no flesh would be saved; but for the elect’s sake, whom He chose, He shortened the days.
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.