'Letter' in the Bible

not to be quickly unsettled or alarmed either by a [so-called prophetic revelation of a] spirit or a message or a letter [alleged to be] from us, to the effect that the day of the Lord has [already] come.

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

Now if anyone [in the church] does not obey what we say in this letter, take special note of that person and do not associate with him, so that he will be ashamed and repent.

I, Paul, write you this final greeting with my own hand. This is the distinguishing mark in every letter [of mine, that shows it is genuine]. It is the way I write [my handwriting and signature].