In 1 John 2.9-11, John uses adelphos, or brother. Researching that, I came across The Outline of Biblical Usage by Larry Pierce which says adelphos is sometimes used to mean countryman and co-employee. I have not found instances where those other meanings are explicit, but is it appropriate to use those meanings in 1 John 2? Honestly, I don't know but am open to arguments pro and con. While I realize that verse 13 indicates who the audience is, it does not indicate if John was restricting his commandment to be applied by Christians to other members of the audience exclusively. It would certainly be easier on us to believe the commandment is only relevant to those we know in a Christian context, and not to people in the workplace, etc. But that would not match the parable of the good Samaritan. So I'm not sure John was limiting the application or the meaning of adelphos. But he could have been doing the latter. I just don't know. I guess a broader question might be: when are Pierce's extended meanings ever applicable?
The word ἀδελφός (adelphos) occurs well over 300 times in the NT for which BDAG has two basic meanings:
(1) a male from the same womb as the reference person, brother, eg, Matt 1:2, 11, 4:18, 21, Gal 1:19, etc. … Hence there is no doubt that in Luke 21:16 that the plural "adelphoi" = brothers and sisters …
(2) a person viewed as a brother in terms of a close affinity, brother, fellow member, member, associate
(a) one who shares beliefs eg, matt 12:50, Mark 3:35 …
(b) a compatriot Lev 10:4, 15:3, 12, 17:15, Acts 2:29, 3:17 etc.
(c) without reference to a common nationality or faith, neighbor 9or an intimate friend), eg, Matt 5:22, 7:3, etc.
(d) Form of addressused by a king to persons in very high position - (used in classical, non-Biblical literature)
I have only included the highlights from BDAG to provide the general idea. For more detail consult BDAG. However, it appears correct that "adelphos" can, indeed, be used in the sense of fellow countryman or compatriot, or even close associate.
In I John 2:12, the very next verse, John speaks to 'little children'.
In such a context, it is my own understanding that the 'brother' referred to in the previous few verses would be a sibling. Or like a sibling - namely a brother in Christ.
To hate one's own Christian brother is a terrible thing. I believe that is what John is talking about.
The definition is found in Greek Lexicon de Thayer: Thayer’s Greek Lexicon Strong’s NT 80: ἀδελφός ἀδελφός, (οῦ, ὁ (from ἆ copulative and δελφύς, from the same womb), however, the word was used for the father Jacob in Genesis 37:3-4 with four different wives (septuagint). There was an advance of meaning of the word for Jacob. All these were brother cousins (adelphos) by the same father.
What's the meaning of brother in 1 John 2.9-11?
In context of John's writings,the expression "brothers" refers to the anointed brothers of Christ, more commonly known as "saints" they are also referred to as "the least of these brothers or sisters of mine.
Matthew 25:40 NET
40" And the king will answer them,[d] ‘I tell you the truth,[e] just as you did it for one of the least of these brothers or sisters[f] of mine, you did it for me."