In 1 Thessalonians 4:9–10 Paul encourages the church at Thessalonica to pursue “brotherly love.” What is brotherly love? The noun used here (φιλαδελφία) was only used for literal family relationships before the Christian community began to use it as a metaphor for members of their community (EDNT, 4:434). The only exception appears to be 2 Macc 15:14, the word appears to refer to a fraternal relationship of all Israel. In 4 Macc 13:23, 26, 14:1 the word refers to the mutual love between seven brothers who all suffer instead of reject their Jewish traditions.
The Greeks considered the relationship between brothers to be of primary importance, Plutarch used the term “brotherly love” to describe the proper relationship between brothers.
Plutarch, De fraterno amore 2 …where there is an unanimous accordance amongst brothers, the family thrives and flourishes, and friends and acquaintance, like a well furnished choir, in all their actions, words, and thoughts maintain a delightful harmony. “But jarring feuds advance the worst of men.”
Plutarch, De fraterno amore 15 Brothers should not be like the scales of a balance, the one rising upon the other’s sinking; but rather like numbers in arithmetic, the lesser and greater mutually helping and improving each other.
Plutarch, De fraterno amore 21 Again, it is highly commendable in him to have the highest esteem and honor for his brother’s wife, reputing and honoring her as the most sacred of all his brother’s sacred treasures, and thus to do honor to him
Based on the teaching of Jesus, the earliest believers referred to themselves as “brothers and sisters.” In Mark 3:3 Jesus indicates that his “brothers and sisters” are those who hear and obey his words. If those who followed Jesus faced rejection from their families, it is possible that Jesus intended his followers to be a new “family.”
On the other hand, the family of Jesus may be an allusion to the larger theme of a New Israel among the followers of Jesus. In Acts 2:29 Peter addresses a Jewish crowd as “brothers,” meaning “fellow Jews.” So too Paul in Acts 22:1; 23:1.
Paul’s use of the term “brother” and “brotherly love” bears additional theological weight. By accepting Christ, we are adopted into the family of God, God is our father. This makes each person that has accepted Christ as their savior a brother or sister in Christ.
This new family in Christ is the foundation for many of Paul’s commands (cf., Rom 12:10; other Christian ethical instruction begins the same way (Heb 13:1; 1 Pet 1:22; 2 Pet 1:7; 1 Clem 48:1). He urges his readers to please God by treating each other like brothers and sisters.
If the church lives in brotherly love, then the father is pleased and honored. For the Greek world, nothing dishonors the parent more that children who do not display proper affection for one another and feud. If Plutarch could say “jarring feuds advance the worst of men,” how might he describe the sort of angry disputes which plague most modern churches?