Romans 8:29, NKJV: "For whom He foreknew, He also predestined to be conformed to the image of His Son, that He might be the firstborn among many brethren [brothers]."
It may look the Son is considered one of the brethren. But is it true?
However it is clear that, when Paul used the word "brethren" (brothers, brother) without further identification (as in 1 Corinthians 9:5, Galatians 1:19 & Romans 9:3) (81 times in the seven deemed authentic epistles), he meant all the times "Christian(s)".
Note: Carrier wrote: "the evidence shows Christians were all called brothers ..." (Ref: here).
Now if we replace "brethren" by "Christians" in Romans 8:29, we get:
"For whom He foreknew, He also predestined to be conformed to the image of His Son, that He might be the firstborn among many Christians [brethren]."
Gone is the (wrongly) perceived notion of a spiritual link as brothers between Jesus (the Son) and the "brethren". And if Paul wanted to express a brotherly relationship between the Son and the brethren/brothers, he would have written:
"... that He might be the firstborn of [among] many brethren."
"... that He might be the firstborn among his many brethren."
The Greek words used in the verse for "many" & "brethren" are in the dative case (while"firstborn" is accusative). That means "many brethrens" is an indirect object to "firstborn", certainly not connected by any familial (spiritual) relationship.
If Paul thought of his Christians as being "brothers of the Son/Lord/Christ/Jesus", he would have used the genitive case instead
. But he did not. For further explanations, see this website
Note: for all the four occurrences of "sons of God" in the Pauline epistles (Romans 8:14, 19 & 9:26 and Galatians 2:20), "God" is always in the genitive case.
Also in the two occurrences of "brother(s) of the Lord" (1 Corinthians 9:4 and Galatians 1:19), "Lord" is in the genitive case.
Certainly Romans 8:29 cannot be used as evidence for Paul considering Christians as brothers of Jesus, the Son of God, in any ways. Actually, what is shown here looks like a denial.