What would they think?

So when Paul write the Thessalonians

"Let no one deceive you in any way. For that day will not come, unless the rebellion comes first, and the man of lawlessness is revealed, the son of destruction, who opposes and exalts himself against every so-called god or object of worship, so that he takes his seat in the temple of God, proclaiming himself to be God."

and there is a standing temple, wouldn't they think that it is the existing temple of which Paul is speaking.

Futurist would say that we have not seen this yet, that it will take place in a temple yet to be built. So years later, did the Thessalonians, when they saw the destruction of the standing temple, release they hadn't seen this man of lawlessness. Did they reason "Well, we didn't see this guy so I guess God will build another temple at some point in the future. He did it in the past."

I doubt it. This was the temple that the Messiah was presented in, taught in, etc. I think this passage has application to the people to whom it was written.