All is emphatic. The Greek has the conjunction “for” (omitted in Good News Translation), which explains the relation of 4a to the preceding verse. People who belong to the light, who belong to the day translates a Greek text which is literally “sons of light you are, and sons of day.” The expression “sons” goes back to a Hebrew idiom used to identify personal characteristics (cf. Acts 4.36, where the name Barnabas is translated literally “son of encouragement,” Good News Translation “One who Encourages”). The literal translation, followed by most traditional and many modern versions, has little meaning or impact in non-Semitic languages.
We do not belong to the night or to the darkness emphasized verse 5a by putting the same idea in negative form. It also marks the inclusion of Paul and his companions by the change from you to we, which is maintained until verse 11. Bijbel in Gewone Taal attractively translates “we have nothing to do with the darkness of night.” We, or course, includes both senders and readers of the letter.
This verse seems quite simple in an English translation such as Good News Translation, but the concepts are extremely difficult to communicate effectively in some languages. It may not make sense to say people who belong to the light or we do not belong to the night. It is possible to speak of an object as belonging to a person, but how can a person belong to light? Nevertheless, in order to introduce the contrasts occurring in verse 6-8, at least reference to “light” and “darkness” and “day” and “night” is required in verse 5. In some cases one may translate “all of you are people who live just as though everything were done in the light of day,” or “… do everything as though it were done in the light, that is in the daytime.” The second part of verse 5 is simply a negative reflection of the first part, and can be translated in some languages as “we do not live like people who always carry on at night or in the darkness,” “… who do what they do at night or in the darkness,” or “… when no one can see.”
Quoted with permission from Ellingworth, Paul and Nida, Eugene A. A Handbook on Paul’s First Letter to the Thessalonians. (UBS Handbook Series). New York: UBS, 1976. For this and other handbooks for translators see here .