Because in my view, the early Church Fathers were correct in concluding that His salvific role requires His divinity. Few Christians today dwell on the question of the necessary qualifications for playing that salvific role. We might ask the question this way: what must be the victim’s nature in order to pay the price for mankind's sin? Would a sinless man fill the bill here? Or must the victim have been something more – and if so, how much more?
The early Church Fathers wrestled with this. They ultimately concluded that the ransom price was high indeed. They ultimately concluded that the victim must be divine. It was a conclusion about how to reverse the curse and repairing the rift between God and man. The blood of God Incarnate was required to wash away mankind's sin. No lesser solvent would do.
What helped me understand this was an exegesis of Paul’s use of the pleroma – translated as “fullness” – twice in Colossians and twice in Ephesians. In Colossians, Paul attributes to Christ the fullness of God, Colossians 1:19 (“For in him all the fullness of God was pleased to dwell”); Colossians 2:9 (“For in him the whole fullness of deity dwells bodily”). And in Ephesians, he hints at that same fullness present in Christ being transferable to humanity, Ephesians 3:19 (“and to know the love of Christ that surpasses knowledge, that you may be filled with all the fullness of God”); Ephesians 4:13 (“until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ”). For Paul, there was at least some sense in which mankind could, through Christ, attain to the divine.
The early Church Fathers who worked out the Trinity doctrine thought so too. Athanasius’ fourth century work On the Incarnation famously states “God became man that man might become God.” He wasn’t blaspheming that we would all become equal to God. He was commenting on restoration of mankind’s union with God. Other Patristics did to. Clement of Alexandria, in the first chapter of his Exhortation to the Heathen, writes “I say, the Word of God became man, that you may learn from man how man may become God.” Origen, in the third chapter of his Contra Celsus, writes “from Him there began the union of the divine with the human nature, in order that the human, by communion with the divine, might rise to be divine.”
I agree with them. You are free to disagree, and I'll respect your view -- as I hope you can respect mine.
Yes-- while a man was raised up, a divine spirit descended.