Why are Christians called 'brethren'?

The term 'brethren' signifies that Christians are not only individuals but members of a larger family united under God's fatherhood. In Galatians 4:26, Paul writes, 'the Jerusalem which is above is free, which is the mother of us all,' indicating that believers are born anew in Christ and stand together as children of God. This identity comes with the understanding that true fellowship exists among those who share the common experience of salvation through Christ. As believers gather, they embody the reality of being 'brethren,' supporting one another in the spirit of love and grace, and encouraging one another toward holiness and good works, rooted in their shared relationship with the Father.