29 For whom he foreknew, he also predestinated to be conformed to the image of his Son, that he might be firstborn among many brethren;
Who did God foreknow?
The WORD who would become flesh.
Who was conformed to be the image of God Son?
the WORD made flesh You TODAY are my Begotten Son.
Who is the firstborn among many brethren?
the WORD made flesh.
The entire verse is about JESUS, the WORD made flesh.
For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that
he ( Jesus ) might be the firstborn among many brethren.( The children of God ) [30] Moreover whom he did predestinate,
them he also called: and whom he called, them he also justified: and whom he justified, them he also glorified.
It's this Jesus first by himself alone and then those who are believers in him are included as the elect" the body of Christ "
Galatians 3:16, 26-29 KJV
[16] Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but
as of one, And to thy seed, which is Christ.
...[26] For ye are all the children of God by faith in Christ Jesus. [27]
For as many of you as have been baptized into Christ have put on Christ. [28] There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female:
for ye are all one in Christ Jesus. [29] And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
Christ is the singular heir who is sharing his Inheritance with believers in him the singular elect that we are elect in him
Hebrews 2:9-11, 13 KJV
[9] But we see Jesus, who was made a little lower than the angels for the suffering of death, crowned with glory and honour; that he by the grace of God
should taste death for every man. [10] For it became him, for whom are all things, and by whom are all things, in
bringing many sons unto glory, to make the captain of their salvation perfect through sufferings. [11] For both he that sanctifieth and they who are sanctified are all of one: for which cause he is not ashamed to call them brethren,
[13] And again, I will put my trust in him. And again,
Behold I and the children which God hath given me.