In the letter that I wrote you I told you not to associate with immoral people.
I wrote unto you in an epistle not to company with fornicators:
I wrote you in my [previous] letter not to associate [closely and habitually] with unchaste (impure) people–
I wrote unto you in my epistle to have no company with fornicators;
I wrote to you in my earlier letter not to associate with sexually immoral people.
As I have written to you in an epistle: "Do not associate with fornicators,"