The impassibility of God pertains to the question of whether God can suffer or be acted upon by any created thing. For God to change would require that God become something other than he is. God would either have to become perfect (at least for a while), fall from perfection and remain imperfect, or vacillate between perfection and imperfection. It would, also, be possible that God would never reach and maintain divine potentiality.
God doesn’t have potentiality. His attributes cannot develop or diminish. It is impossible for God to become more or less of what his attributes contemplate. God cannot reach his potential or forgo it. Rather, God exists eternally in his divine potential. That’s why theologians speak of God’s actuality. God is pure act. For instance, God is maximally loving in and of himself. God is forever unchanging in his love. Better yet, God’s love is unchanging. Or even better still, Love is unchanging (because God is simple and love is a divine attribute. God is love).
God’s unchanging character (his immutability) does not undermine his affections. Rather, God’s immutability establishes his affections. It is because God is impassible that he shows forth his pure affections. Because God is unchanging in his mercy, he expresses or releases the actuality of his mercy toward those who by grace seek it through his Son. God’s maximum mercy doesn’t get turned on. Rather, it gets willfully directed and from our perspective released at divinely appointed times.
Unhappy alternatives
Some have suggested that God decreed that he be acted upon. Others have mused that God willingly takes on properties that allow him to change in relation to his covenant promises. Both positions are attempts to affirm divine impassibility while trying to avoid attributing too many biblical texts to anthropomorphic language. Notwithstanding, both theories are outside the pale of orthodox Christianity since in both cases God’s nature would be mutable even if by decretive condescension.
When God takes aim with his affections, he does so consciously and without change. He is merely responding (not reacting) to changes outside himself – changes he has ordained in others. God does not change, but the ordained states of affairs do change according to God’s will and providence, to which God responds appropriately according to his unchanging attributes. Accordingly, God’s immutable attributes ground his dynamic responses in relationship to his ever changing creation.
Without impassibility, no incarnation
For man to be saved he needs a mediator who would learn obedience through suffering. Redeemable creatures need a high priest who can be tempted in all ways like them but without sin. Yet God cannot learn obedience nor be tempted. Yet if God were passible, he could have ordained that he be tempted and suffer apart from the incarnation of the Son.1 Therefore, if God is passible, the hypostatic union is unnecessary. A body for the Son would not have been prepared for him. There would have been no need for Messiah. Yet it is precisely because God is impassible that the cup of wrath could not pass from Jesus as he petitioned. In other words, it would have been possible for the cup to have passed if God could suffer. Therefore, Jesus’ petition in his high priestly prayer affirms God’s impassibility, hence the need for a suffering Messiah.2
In discussions over impassibility we must stay clear of two ditches – deism and process theology, divine apathy and divine vulnerability. However, we must be sure that in our avoidance of the one, we don’t stumble into the other.
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1I’m speaking in a reductio ad absurdum fashion, since obviously if the incarnation were not necessary to save because God is passible, then our reconciliation would be to a God who is less than eternally perfect. Christianity unravels.
2Redemption required the Son of God to empty himself, not by subtraction but addition. In the incarnation the Son remained God. And although he remained a divine person, he added to himself our humanity yet without confusion, change, division or separation. One person was now two inseparable beings. Through the incarnation God the Son could now suffer in his humanity.
In predicating to the Son his actions, we must distinguish which nature is the source. The Son thirsts in his humanity whereas he calms the storm in his divinity. Divine acts can be done by the Son through his human nature but not by his human nature. All acts, whether divine or human, may be predicated to the Son, but not all acts of the Son may be predicated to either nature.
Philosophical Theology 
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