It was predicated on Christ's life of perfect obedience, and on his bearing a nature identical to our own.
This fundamental principle of the atonement is dealt with no less than six times in the apostolic writings, and the exposition they give on each and every occasion, whether speaking to Jews or Gentiles, is exactly the same:
14Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil;
15And deliver them who through fear of death were all their lifetime subject to bondage.
Christ was made of the very same flesh and blood as ourselves, in order that through his death he might destroy the power of death, and deliver those subject to bondage.
So, according to Paul here, it was imperative that Christ be of the same flesh and blood as we ourselves, in order to effect the atonement.
Thus we see that the deliverance of those subject to bondage was predicated on Christ's mortality. Of his 'Divinity', nothing is said...