1Th did not dispell the flawed beliefs about the Parousia. On the contrary, they gained in strength. Some were even saying that the Day of the Lord [had] already come (v. 2).
The source of the deception (2:3) is unclear. Perhaps something had happened (so Bruce, 164). Perhaps it was a prophetic message, a nonecstatic spoken word, or a misreading of 1Th which fanned this “radical millenarianism” (see Jewett, passim). Paul finds such a misinterpretation so incredible that he wonders whether the source may be a forgery. At the end of this letter he takes deliberate steps to preclude that possibility; at this point he sets the record straight about the timing of the Parousia.