'Letter' in the Bible

that you not be quickly shaken from your composure or be disturbed either by a spirit or a message or a letter as if from us, to the effect that the day of the Lord has come.

So then, brethren, stand firm and hold to the traditions which you were taught, whether by word of mouth or by letter from us.

If anyone does not obey our instruction in this letter, take special note of that person and do not associate with him, so that he will be put to shame.

I, Paul, write this greeting with my own hand, and this is a distinguishing mark in every letter; this is the way I write.