Paul seems to have been writing to a mixed group which in included both Jews and former pagans (compare verse 9 and Acts 17.1-3). This makes it all the more remarkable that he should call them all brothers, as he does repeatedly in this letter and elsewhere. In the ancient world, it was almost unheard-of for anyone to call a foreigner “brother,” but in the Christian family it was common from the beginning. It makes no difference to Paul that not long before this some of his readers had been fellow Jews and others had been pagans; now, without any distinction, they are all brothers. Though in many languages the Christian community refers to all its members by a word which more of less literally means “brothers” or even “brothers and sisters,” in some instances a term for broader kinship is employed, for example, “relatives” or “clansmen,” since this would be the only appropriate way in which one could address a larger group with which one is intimately associated.
We still includes, of course, Silas and Timothy, but not the Thessalonians. This is the case in the entire section until verse 10, where the first person plural becomes inclusive; that is, the expression us there includes the Thessalonian Christian as well as Paul and his companions.
Good News Translation rightly makes explicit two things which are only implicit in the Greek: (1) the connection between we know and God loves you (literally, “we know, brothers beloved by God”) and (2) the component of meaning expressed by to be his own. Good News Translation, like Revised Standard Version New English Bible etc., avoids the traditional word “election,” which now has political associations, and translates this noun by a verb, since it clearly refers to something God does. But it is not enough to translate simply “we know … that he has chosen you” (Revised Standard Version cf. New English Bible Biblia Dios Habla Hoy Bijbel in Gewone Taal Translator’s New Testament), because the word Paul uses has clear associations in the minds of his readers with God’s choice of Israel to be his special people and to fulfill a special mission. God’s choice of the Thessalonian believers must be brought out by some such phrase as to be his own, “to belong to him” (Bible en français courant), “to belong to his people” (Die Bibel im heutigen Deutsch), or “for a special purpose” (Phillips).
Has chosen you to be his own may be translated more or less literally as “has selected you out so that you could belong to him,” but it is sometimes better expressed in more figurative language, for example, “has chosen you and put his mark upon you,” “has labeled you as his possession,” or “has selected you and marked you as his.”
Expressions corresponding to God loves you frequently reflect terms which denote the love of parents for children, but in some languages figurative expressions are used, for example, “God holds you in his heart,” “God’s heart goes out to you,” or “God treasures you very much.”
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 .