In many New Testament letters, the readers are addressed as “brothers”. See for example Romans 1:13, 1 Corinthians 1:10, Galatians 1:11 and 2 Peter 1:10. In the book of Acts, we read that this was already common practice among the apostles and in the early church (e.g. Acts 1:15-16, Acts 2:37, Acts 15:3, Acts 15:23).
Some Bible translations write “brothers and sisters” here, to make clear that both men and women are addressed by this general term. It is clear that not all these people are from the same family. So why do they call each other “brothers and sisters”?
Jesus defines who are His brothers and sisters
In the Gospels, we read how Jesus gives a new definition of family membership. He is told that His mother and brothers are seeking Him. They are His “real” family members, but Jesus answers: “Who are My mother and My brothers? […] Whoever does the will of God, he is My brother and sister and mother” (Mark 3:32-35).
Jesus defines brotherhood not by natural relationships, but by spiritual connections. This does not mean that natural family members are not important (see e.g. 1 Timothy 5:8). But it does mean that the spiritual relationship is even stronger.
Christians are all children of one Father
Believers are brothers and sisters because they all have a special relationship with God the Father. Romans 8:14-16 explains: “For all who are led by the Spirit of God are sons of God. For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, Abba! Father! The Spirit Himself bears witness with our spirit that we are children of God.”
The same chapter says: “And we know that for those who love God all things work together for good, for those who are called according to His purpose. For those whom He foreknew He also predestined to be conformed to the image of His Son, in order that He might be the firstborn among many brothers” (Romans 8:28-29). Jesus Christ is the only Son of God. But God adopts humans into His family, so that they all become spiritual brothers and sisters.
Being brothers and sisters in Christ gives us a certain responsibility for each other. We are to love others as ourselves, but our spiritual family has a special place. As Galatians 6:10 says: “So then, as we have opportunity, let us do good to everyone, and especially to those who are of the household of faith”.
We are called to brotherly love (John 13:34, Romans 12:10, Hebrews 13:1). We are called to sympathize with each other’s sorrows and to live in unity (1 Peter 3:8). Together we are the one worldwide church, the bride of Christ.