'Letters' in the Bible

Are we beginning to recommend ourselves again? Unlike some people, we do not need letters of recommendation to you or from you, do we?

Now if the ministry of death that was inscribed in letters of stone came with such glory that the people of Israel could not gaze on Moses' face (because the glory was fading away from it),

I do not want you to think that I am trying to frighten you with my letters.

For someone is saying, "His letters are impressive and forceful, but his bodily presence is weak and his speech contemptible."