In Hebrew 5:9, what changed about the Son's perfection? In the book of Hebrews, we read:


  8 Son though he was, he learned obedience from what he suffered 9 and, once made perfect, he became the source of eternal salvation for all who obey him 10 and was designated by God to be high priest in the order of Melchizedek. Hebrews 5:8-10 (NIV)


Here, the writer to the Hebrews describes God the Son as being "made perfect". The simple reading of this would imply that he was imperfect before this event, but that would contradict my understanding of the rest of scripture.

How would a reformed protestant understand the idea of God the Son being "made perfect"?
Calvin has this to say about Hebrews 5:9, using the word 'sanctified' for 'perfect' :


  Sanctified suits the passage better than "made perfect." The Greek word teleiotheis means both; but as he speaks here of the priesthood, he fitly and suitably mentions sanctification. And so Christ himself speaks in another place, "For their sakes I sanctify myself." (John 17:19.) It hence appears that this is to be properly applied to his human nature, in which he performed the office of a priest, and in which he also suffered. [90]


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Gill adds the word 'completed':


  And being made perfect,.... In his obedience, through sufferings; having completed his obedience, gone through his sufferings, and finished his sacrifice, and being perfectly glorified in heaven:he became the author of eternal salvation unto all them that obey him


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Matthew Henry uses the word 'consecrated' in related to 'perfect' :


  (2.) By these his sufferings he was made perfect, and became the author of eternal salvation to all who obey him, v. 9. [1.] Christ by his sufferings was consecrated to his office, consecrated by his own blood. [2.] By his sufferings he consummated that part of his office which was to be performed on earth, making reconciliation for iniquity; and in this sense he is said to be made perfect, a perfect propitiation. 


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