"I argued that all Christians were “brothers of the Lord” because:
(a) they were all adopted sons of God,
(b) Jesus was an adopted son of God,
(c) that by definition made them all the adopted brothers of Jesus;
(d) Christians called each other brother, therefore they would have called each other brothers of Jesus, too.
I also showed
(e) that they believed Jesus had explicitly called them his brothers
(f) they explicitly said Jesus was only “the firstborn among many brethren."”
Then Carrier wrote the following:
"Another important point I made is that Jesus became Lord at his adoption, so Christians would be brothers of the Lord specifically, a uniquely Christian concept (and one that could only have been uttered after the origins of Christianity; e.g., even if James was the biological brother of Jesus, he would never have been called “the brother of the Lord” until Christians invented that phrase for him)."
First, we have no way to know what the Christians thought of themselves. What we can conclude is only what Paul wanted his audience to believe. And he never wrote his Christians saw themselves as "brothers of the Lord" or even "sons of God".
For (a): it was Paul's claim, not the Christians' one. When did that come?
Christians being sons of God appears in 2 Corinthians 6:18, Galatians 3:26, 4:5-6 & Romans 8:14, 8:19, 9:26 but never in 1 Corinthians written earlier.
And I am certain the passage (6:14-7:1) including 2 Corinthians 6:18 is a late interpolation (see full explanation here
What follows are extracts (slightly edited):
"The preceding paragraph is likely a late addition. It certainly does not fit well in the passage and appears to be an outright insertion. The style is also unlike Paul's. Paul never mentioned Belial, only Satan (1 Thessalonians 2:18, 1 Corinthians 5:5, 7:5, 2 Corinthians 2:11, 11:14, 12:7 and Romans 16:18). Also the Greek words for "yoked", "fellowship","part", "agreement", "Almighty" & "filthiness" and the expression "sons and daughters" appears only here in all the Pauline epistles.
Note: verse 18 is obviously drawn from 2 Samuel 7:14, but, in both the Hebrew and the Septuagint, there is no mention of "daughters" and "son" is singular (referring to Solomon): "I will be to him a father, and he shall be to me a son."
And then, it is conflicting with the following verses, where co-existence with unbelievers (even as a spouse!) is accepted: 1 Corinthians 5:9-10 ... 1 Corinthians 10:27-28 ... and 1 Corinthians 7:12-13 ..."
"sons of God" appears first in (written later) Galatians
(3:26), after a long discussion in order to justify this title (among other things), suggesting Paul's Christians were not aware of it before.
So there is a very strong possibility the converts of Corinth were not aware Paul calling Christians (adopted) "sons of God" when 1 Corinthians was written.
For (b): Carrier's argument about "Jesus was an adopted Son of God, resting on Romans 1:4, is greatly flawed:
"and declared to be the Son of God with power according to the Spirit of holiness, by the resurrection from the dead."
Paul did not mean Jesus was adopted then by God, but just that Jesus was proven to be the Son by his resurrection. How to prove it?
According to Paul, before coming to earth (or in the sky, as Carrier wants us to believe), Jesus is already the Son of God (and pre-existent):
Galatians 4:4 "But when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law,"
Romans 8:3 "... God did by sending His own Son in the likeness of sinful flesh ..."
Other instances of the pre-existence of Jesus as a heavenly divinity: 1 Corinthians 8:6 & 10:3-4
So again, Carrier's claim about the adopted Son of God rests on a very weak foundation.
To make it worse for Carrier, I am certain the two occurrences of "Son of God" in 1 Corinthians are parts of interpolated passages (1:4-9, 15:23-28), as also the one in 1 Thessalonians (1:10), the only epistle written earlier than 1 Corinthians. See explanations here
But let's say, in '1Thessalonians' and '1Corinthians', Paul was unlikely to mention Jesus as "the Son", because he wrote: "God the Father and the Lord Jesus Christ" (1 Thessalonians 1:1) "our God and Father" (1 Thessalonians 1:3,3:13) "our God and Father himself and our Lord Jesus" (1 Thessalonians 3:11) "For even if there are so-called gods, whether in heaven or on earth (as indeed there are many gods and many lords, [for Paul, as it seems here, "lords" are not "gods"] yet for us there is one God, the Father ... and one Lord Jesus Christ ..." (1 Corinthians 8:5-6a) "the heavenly man [Jesus]" (1 Corinthians 15:48,49)
So it is very unlikely the Christians of Corinth thought that Paul acknowledged then (when 1 Corinthians was written) Jesus as "Son of God", even less an adopted "Son of God".
For (c): in view of my conclusions for (a) & (b), the chances of (c) being true are close to zero.
For (d): certainly Paul called these Christians "brothers". The rest is unproven (with evidence against!).
For (e): where did Carrier get that?
Paul, not Christians, said that and as I demonstrated in my previous post #80
, he made a point NOT to have the Christians then as (spiritual) brothers of Jesus.
Finally, I find myself in agreement upon the last sentence I quoted from Carrier:
"even if James was the biological brother of Jesus, he would never have been called “the brother of the Lord” until Christians invented that phrase for him"