A.T. Robertson Commentary Romans 8:29

"For whom he foreknew, he also foreordained [to be] conformed to the image of his Son, that he might be the firstborn among many brethren:" — Romans 8:29 (ASV)

Foreknew (προεγνω). Second aorist active indicative of προγινωσκω, old verb as in Ac 26:5. See Ps 1:6 (LXX) and Mt 7:23. This fore-knowledge and choice is placed in eternity in Eph 1:4.

He foreordained (προωρισεν). First aorist active indicative of προοριζω, late verb to appoint beforehand as in Ac 4:28; 1 Corinthians 2:7. Another compound with προ- (for eternity).

Conformed to the image (συμμορφους της εικονος). Late adjective from συν and μορφη and so an inward and not merely superficial conformity. Εικων is used of Christ as the very image of the Father (2 Corinthians 4:4; Colossians 1:15). See Php 2:6f. for μορφη. Here we have both μορφη and εικων to express the gradual change in us till we acquire the likeness of Christ the Son of God so that we ourselves shall ultimately have the family likeness of sons of God. Glorious destiny.

That he might be (εις το εινα αυτον). Common idiom for purpose.

First born among many brethren (πρωτοτοκον εν πολλοις αδελφοις). Christ is "first born" of all creation (Colossians 1:15), but here he is "first born from the dead" (Colossians 1:18), the Eldest Brother in this family of God's sons, though "Son" in a sense not true of us.