5:9-13 Christians are to avoid familiar converse with all who disgrace the Christian name. Such are only fit companions for their brethren in sin, and to such company they should be left, whenever it is possible to do so. Alas, that there are many called Christians, whose conversation is more dangerous than that of heathens!
11. But now—"Now" does not express time, but "the case being so," namely, that to avoid fornicators, &c., of the world, you would have to leave the world altogether, which would be absurd. So "now" is used in Heb 11:16. Thus we avoid making the apostle now retract a command which he had before given.
I have written—that is, my meaning in the letter I wrote was "not to keep company," &c.
a brother—contrasted with a "fornicator … of the world" (1Co 5:10). There is less danger in associating with open worldlings than with carnal professors. Here, as in Eph 5:3, 5, "covetousness" is joined with "fornication": the common fount of both being "the fierce and ever fiercer longing of the creature, which has turned from God, to fill itself with the inferior objects of sense" [Trench, Greek Synonyms of the New Testament]. Hence "idolatry" is associated with them: and the covetous man is termed an "idolater" (Nu 25:1, 2). The Corinthians did not fall into open idolatry, but ate things offered to idols, so making a compromise with the heathen; just as they connived at fornication. Thus this verse prepares for the precepts in 1Co 8:4, &c. Compare the similar case of fornication, combined with a similar idolatrous compromise, after the pattern of Israel with the Midianites (Re 2:14).
no not to eat—not to sit at the same table with such; whether at the love-feasts (agapæ) or in private intercourse, much more at the Lord's table: at the last, too often now the guests "are not as children in one family, but like a heterogeneous crowd of strangers in an inn" [Bengel] (compare Ga 2:12; 2Jo 10, 11).