The Scriptures employ various terms to convey the relationships Christians have with one another. One of the most common ways the Bible does this is using familial terms. Consider the statements Paul made at the end of his epistles, such as, “All the brethren greet you” (1 Corinthians 16:20). Often the term “brethren” appears in Paul’s writings to convey his dear relationships to other Christians.
The metaphor is based on the literal, physical, flesh-and-blood relationships we have. It is natural to have an affection for our own; something is seriously wrong with those “without natural affection”—who don’t care for their own family (Romans 1:31; 2 Timothy 3:3; 1 Timothy 5:8). Paul referred to a literal family when he wrote of “the brethren of the Lord” (1 Corinthians 9:5). Paul was thinking “of James, and Joses, and Juda, and Simon” (Mark 6:3), Jesus’ half-brothers in the family of Joseph and Mary. During His earthly ministry, Jesus’ brethren did not believe in Him (John 7:5). In regard to fraternity, Jesus gave priority to those who had the spiritual interests aligned with His own: “And it was told him by certain which said, Thy mother and thy brethren stand without, desiring to see thee. And he answered and said unto them, My mother and my brethren are these which hear the word of God, and do it” (Luke 8:20, 21). It seems from references in the rest of the of the New Testament that at least some the physical family of Jesus believed and became part of the spiritual family of the church. James and Jude wrote NT books that bear their names. After the ascension of Jesus to the Father, His mother Mary and His brethren continued in one accord with the disciples (Acts 1:14).
Under the OT system, every Jew looked back to Abraham as their common ancestor (Matthew 3:9). This is why Ananias called Saul “Brother” in Acts 9:17. It was not because Saul was already saved—and thereby they were spiritual brothers—for Saul had yet to wash away his sins in baptism (Acts 22:16). Ananias and Saul were Jews, descendants of Abraham. Once Saul was baptized, he was born again into the spiritual family of God (John 3:3, 5). At that point Ananias and Saul were not only Jewish brothers, but more importantly brothers in the Lord.
In the church, the family or household of God (1 Timothy 3:15), there is a Father (1 John 3:1) who has begotten us by His word (James 1:18; 1 Peter 1:23). As children of the same Father, Christians are siblings who must behave in a way that glorifies the Father. We are commanded not be at the family fellowship table with one who is called a brother that does not live according to the Scriptures, unless the erring brother repents (1 Corinthians 5:11). Jesus, in a sense, is an older brother, because He is the firstborn among many brethren (Romans 8:29). He is not ashamed to call us brethren because of our common experiences; He is one with those He sanctifies (Hebrews 2:11).
As Christians, we should add “brotherly love” to our hearts, by being kind and affectionate toward each other (Romans 12:10; 2 Peter 1:7). God has showed us how to love each other (1 Thessalonians 4:9). When we were so influenced by God’s love that we decided to purify our souls by obeying the truth, we did so unto unfeigned (or genuine) love of the brethren; therefore, we should love one another fervently from the heart (1 Peter 1:22). Where brotherly love exists, we must let it continue (Hebrews 13:1).