In Acts, Paul refers to “Brothers. ” In the context of the verses he seems to refer to a brother as the son of one's father, while later the context is as specific males belonging to his Christian group. In the gospel, Jesus also seems to offer double meaning to “brother” when His family thought He lost it and came to rescue him. Are “brother” and for that matter “sister” code words with some esoteric significance, known to the first century Christians, that is not rendered in our English translations?
The Greek word for brother is adelphos. There is also a Greek word for sister adelphe. Both words were used in the New Testament to refer to physical flesh-blood brothers or sisters. In Matthew 12:46-50 Jesus was speaking to the crowd and someone told Him his mother and brothers were outside, He responded with,
But He answered the one who was telling Him and said, “Who is My mother and who are My brothers?” And stretching out His hand toward His disciples, He said, “Behold, My mother and My brothers! For whoever does the will of My Father who is in heaven, he is My brother and sister and mother.” (NIV) Matthew 12:48-50
Here adelphos is used of family relationships.
Adelphos was also used to express spiritual brotherhood and warmth towards another person. One great example is found in Acts 23:1, 5 where Paul is before the Sanhedrin Council defending himself. He speaks to a group of men who are members of his family and says,
Paul looked straight at the Sanhedrin and said, “My brothers, I have fulfilled my duty to God in all good conscience to this day.” (NIV) Matthew 12:48-50
Sometimes adelphos was used by Christians to refer to fellow-Christians, such as in Galatians 3:15,
Brothers, let me take an example from everyday life. (NIV) Gal. 3:15
The terms “brothers,” “sisters’ or “brethren” were used by Christians because a man or a woman became part of God’s family when he/she receives or depends on Jesus to forgive his/her sins,
But as many as received Him, to them He gave the right to become children of God, even to those who believe in His name. (NASB) John 1:12
For our citizenship is in heaven, from which also we eagerly wait for a Savior, the Lord Jesus Christ. (NASB) Phil. 3;20
For you are all sons of God through faith in Christ Jesus. (NASB) Gal. 3:26
The terms are also an expression of affection toward fellow Christians. This is the main reason these words were used by the early Christians and by Christians today.
The term “brothers” or “brethren” has always been used by Christians to express affection and to acknowledge their inclusion in a spiritual family – God’s family – which includes the other saints as their brothers and sisters. This relationship or this family exists amongst those whose sins have been forgiven by the blood of Jesus Christ.