I wrote unto you in an epistle not to company with fornicators: I wrote unto you in an epistle - The wisest and best skilled in Biblical criticism agree that the apostle does not refer to any other epistle than this; and that he speaks here of some general directions which he had given in the foregoing part of it; but which he had now in some measure changed and greatly strengthened, as we see from 1 Corinthians 5:11. The words εγραψα εν τῃ επιστολῃ may be translated, I Had written to you in This Epistle; for there are many instances in the New Testament where the aorist, which is here used, and which is a sort of indefinite tense, is used for the perfect and the plusquam-perfect. Dr. Whitby produces several proofs of this, and contends that the conclusion drawn by some, viz. that it refers to some epistle that is lost, is not legitimately drawn from any premises which either this text or antiquity affords. The principal evidence against this is 2 Corinthians 7:8, where εν τῃ επιστολῃ, the same words as above, appear to refer to this first epistle. Possibly the apostle may refer to an epistle which he had written though not sent; for, on receiving farther information from Stephanas, Fortunatus, and Achaicus, relative to the state of the Corinthian Church, he suppressed that, and wrote this, in which he considers the subject much more at large. See Dr. Lightfoot.
Not to company with fornicators - With which, as we have already seen, Corinth abounded. It was not only the grand sin, but staple, of the place.