'Letters' in the Bible

Do we begin again to commend ourselves? or need we, as some others, epistles of commendation to you, or letters of commendation from you?

That I may not seem as if I would terrify you by letters.

For his letters, say they, are weighty and powerful; but his bodily presence is weak, and his speech contemptible.

Let such an one think this, that, such as we are in word by letters when we are absent, such will we be also in deed when we are present.