2 not to be quickly shaken in your mind, and not be troubled, either by spirit, or by word, or by letter as if from us, saying that the day of Christ has already come.
The phrase “not to be quickly shaken in your mind, and not be troubled” refers to being worried or distressed about missing the participation of the rapture.
The phrase “either by spirit, or by word, or by letter as if from us” The word “spirit” indicates a deceiving spirit, demon. The deceiving spirit communicates through false teachers to get their message across to pull people away from the truth.
The church in Thessalonica got such a message. The message was to dismantle their faith by making them believe that Jesus left them behind.
Preterism is the belief that the event of the return of Jesus Christ has indeed already occurred. Their teaching is false, deceptive, and deadly. It will make you believe that there is no mark of the beast so, when it comes you will receive it.
For a debate between a futurist and Preterist click: Preterism Is A Dangerous Theology: A Debate Between A Preterist And A Futurist