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.
nor by word: "By word" indicates someone has received word or messages from the apostles regarding the coming of Jesus Christ being at hand. It seems some of the false teachers pretended to bring a message from the apostle to the Thessalonians, saying the day of Christ was at hand.