Romans 8:29 — For whom He did foreknow, He also did predestinate to be conformed to the image of His Son, that He might be the firstborn among many brethren.
God's foreknowledge marks out the persons, His predestination determines His purposes and acts on their behalf.
Predestinate = mark out beforehand — not because of our faith but as a result of God's eternal purpose in Christ.
Conform — not merely external appearance, but the very nature.
Image — more than resemblance, but representation and manifestation
Firstborn — not birth but priority, dignity, supremacy
God knew us (not just about us) in eternity past — that's foreknowledge. He predestined (marked us out) to take on the nature of His Son.
Predestination is to be distinguished from foreknowledge here,in that God's foreknowledge has special reference to the person foreknown, while predestination has reference rather to that to which the person foreknown is predestinated. — C.R. Stam, The Epistle of Paul to the Romans, page 208.