By calling the Thessalonian Christians brothers,
Paul emphasizes the close relationship which he has with them and which they have with each other. The use of sibling terminology to denote relationships among God’s people goes back to the Old Testament (e.g. Exodus 2:11; Deuteronomy 15:3, Deuteronomy 15:12) and to the ministry of Jesus (Mark 3:34; Matthew 12:49–50; Luke 8:20). It also ties in with the idea of the fatherhood of God (1 Thessalonians 1:1; Romans 8:15). In the midst of a hostile society, the family of believers would have been an essential support system for the new Christians.1,2
4 For we know, brothers loved by God, that he has chosen you,