'Letter' in the Bible

not readily to become unsettled in mind or troubled--either by any pretended spiritual revelation or by any message or letter claiming to have been sent by us--through fancying that the day of the Lord is now here.

So then, brethren, stand your ground, and hold fast to the teachings which you have received from us, whether by word of mouth or by letter.

I Paul add the greeting with my own hand, which is the credential in every letter of mine.