brother, sibling
The word adelphos sits at the intersection of blood and covenant. At its most literal, it means a male sibling, a brother born of the same womb. The Greek prefix 'a' functions as a connective, and 'delphys' means womb. So an adelphos is literally one who shares a womb with you. That biological anchor is important because it tells you what the New Testament writers were reaching for when they stretched this word into new territory.
Paul uses adelphos to address entire congregations, including women. The early church called one another brothers and sisters not as warm religious sentiment but as a factual claim about identity. They were saying: the resurrection of Jesus created a new family, and that family is as real as the one you were born into, perhaps more real.
The word appears over 340 times in the New Testament. Jesus himself redrew the boundary of adelphos in Mark 3:35 when he said that whoever does the will of God is his brother, sister, and mother. That was not poetry. It was a declaration that bloodline no longer defines the innermost circle of belonging. In the book of Acts, the early believers called each other adelphos constantly, a habit that would have shocked their Greco-Roman neighbors, who used family language only for actual family. The church was practicing a new sociology rooted in a new theology: one Father, one Spirit, one body, therefore one family.
Most of us read 'brothers' in Paul's letters and hear a generic greeting, like 'folks' or 'everyone.' I did this for years. I treated it as filler between the theology. But Paul chose a word that means womb-sharers. He was not being casual. He was making a claim that the people sitting across from you in your church share something with you that is more foundational than ethnicity, class, or history. They were born into the same family through the same Spirit. When the church fractures over preferences and offenses, we are not just being difficult. We are denying the womb we share. That is the weight adelphos carries.
From the Greek 'a' (connective prefix meaning 'together') and 'delphys' (womb). The root 'delphys' also gives us 'delphis,' the word for dolphin, named for its womb-like shape. Related forms include 'adelphe' (sister) and 'philadelphia' (brotherly love), which Paul and Peter both use as a moral standard for how believers should treat one another. The semantic family consistently carries the idea of shared origin and mutual belonging.
Where adelphos appears in Scripture, and why each verse showcases it.
For whoever does the will of my Father in heaven is my brother and sister and mother.
Jesus redefines adelphos here in real time, pulling it away from biology and anchoring it in covenant obedience. This is the passage that sets the theological ground for every use of the word in the epistles.
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.
Paul calls Jesus the firstborn among many adelphoi, which means believers are not just friends of God but siblings of Christ. The family language here is doing enormous theological work.
Now concerning brotherly love you have no need for anyone to write to you, for you yourselves have been taught by God to love one another.
Paul uses 'philadelphia,' the compound of adelphos, to describe the kind of love the church should practice, love that is not chosen based on preference but rooted in shared identity.
For he who sanctifies and those who are sanctified all have one source. That is why he is not ashamed to call them brothers.
The writer of Hebrews makes the adelphos language explicit about Jesus himself. Christ is not ashamed to use the womb-word for those he redeems. That declaration deserves to be preached slowly.
If a brother or sister is poorly clothed and lacking in daily food.
James uses adelphos and adelphe together, showing the word functions as a practical category for mutual obligation, not just affection. You owe your brother something concrete.
Words in the same semantic family.
Every video where Adam teaches on this word, in publication order.
This word is studied in depth in the following monthly Bible studies.