A name used for people who belong to God's family. During Jesus's time, Jewish people often called each other "brothers" (Acts 2:29, 37; 7:2; 22:5; 28:21; Romans 9:3). When the first Jewish Christians started their communities, they naturally used this same term to address each other. When they said "brothers," they meant both men and women (Acts 1:15–16; 9:30; 11:1). Many modern Bible translations use "brothers and sisters" instead of just "brothers" to make this meaning clear.
Non-Jewish (gentile) religious groups also called each other brothers. Because of this, when non-Jewish people became Christians, they were comfortable using this term in their churches, too (Acts 17:14; Romans 1:13; 1 Corinthians 1:1, 10). Paul used this term many times in his letters to these churches.
"Brothers" became one of the most common ways to refer to Christians, along with two other terms:
“disciples” (found in the book of Acts) and
“saints” (used as a plural word in Paul's letters and in Revelation).
In the books of James and 1 John, "brothers" is the main term used for Christians.
Early Christians called each individual believer a "brother," and they called the whole group "the brothers." This name showed how close the Christian community was to each other. The relationship between Christians was meant to be as close as family members, and even closer, according to Jesus (Mark 10:23–31).
The books of 1 John and James use this family term to teach that Christians who have more should help Christians who have less (James 2:15; 1 John 3:10–18; 4:20–21). The word "brothers" also shows that all members of the Christian community were equal.