Are you aware that Paul told the Thessalonians the things of II Thess. 2:3-4 while he was with them?
(And of course, this letter was written after he was with them--ca. 52 or so.)
Also, the Thessalonians knew what "withholdeth"--the language of II Thess. 2:6 is evident: "And now ye know . . ."
Would it not be useful to determine what the Thessalonians knew?