The main problem in translating this verse is stylistic. It is caused by the fact that the verse contains a double contrast (1) between verses 14 and 15, (2) between 15a and 15b. The Greek avoids awkwardness by using one word for “but” at the beginning of verse 15, and another and stronger word in the middle. These correspond neatly to Good News Translation‘s but and instead, King James Version‘s “yet” and “but,” and Best’s “however” and “but.” The first “but” is expanded, legitimately but not very neatly, by New English Bible into “I do not mean” (cf. Phillips Jerusalem Bible Barclay Translator’s New Testament). Verse 15b is a positive statement which holds the balance between the negative statement of verses 14b and 15a.

Do not treat him as an enemy may be rendered as “do not act toward him as you would act toward an enemy.” This may be treated idiomatically in some languages as “do not name him as an enemy,” or “do not call him your enemy.”

Warn (a different word from warn in verse 12) may also mean “teach” or “instruct.” The more specific idea of warning fits the context better here. The Greek suggests repeated warnings. Warn him as a brother must be rendered somewhat more explicitly in some languages, for example, “warn him about doing such things in the same way as you would warn your own brother.” On the other hand, the term brother in this context may refer specifically to a fellow believer, and therefore one must say “warn him not to do as he has done, as one who is a fellow believer,” or “… one who also believers in Christ.”

Quoted with permission from Ellingworth, Paul and Nida, Eugene A. A Handbook on Paul’s Second Letter to the Thessalonians. (UBS Handbook Series). New York: UBS, 1976. For this and other handbooks for translators see here .