not to be quickly shaken in your mind, nor yet be troubled, either by spirit, or by word, or by letter as from us, saying that the day of Christ had come.
In order to correct their misunderstanding, Paul assures them the coming of Jesus Christ will not occur until these prophecies are fulfilled. It is not a matter of "if" He will come but "when" He will come. Paul consoles them because he does not want their faith to be overthrown.