Anussava: Hearsay
A report accepted because it was heard. By spirit, by word, by letter: three channels arriving in Thessalonica, and every one of them is someone else's account of a thing nobody in that room has seen. The Kālāmas got the same counsel, to weigh a teaching on its own merits before letting its arrival vouch for it. Frightening news travels fastest, which is the reason it deserves the slowest reception.
Saddhamma-paṭirūpaka: Counterfeit teaching
Doctrine wearing the true teaching's face. "As if from us" is the whole trouble: a letter over Paul's name carries Paul's weight, and whoever wrote it knew that. Counterfeits do not announce themselves as strange; they borrow the trusted voice and keep the familiar cadence, so the test left standing is content, which a forger has to get right line by line and rarely does.
Uddhacca-kukkucca: Restlessness and worry
The hindrance of a mind that rocks and frets. "Quickly shaken" points at speed, the mind moving before it has looked. Water still sloshing in the bucket gives back no reflection, and a congregation already rattled by one report has no steady surface on which to judge it, so settling has to come before deciding.
Yoniso manasikāra: Attention to the root
Taking a thing up from its source. Minds in Thessalonica ran straight to the day of Christ; root attention would have stopped at the paper in their hands and asked who sent it, and why now. Alarm arrives with its conclusion already attached, and the moment of reception is the one place you can still interrupt it.
Sandiṭṭhiko: Visible here and now
A quality of teaching you can check against your own experience. A day that had already come would need no letter to announce it; you would be standing in it. Hold the claim up against what is in front of you and the rumor loses its grip, since anything knowable only through someone else's report keeps you tied to the reporter.