This post is one of a series introduced in the first and last paragraphs here. That should explain why it might be kind of weird.
The concept shared by all sides in the ancient Christological debate, the concept that more than anything else made Christology such an intellectual crux, was the doctrine of God's impassibility, the belief that God could not suffer. This belief, which was practically taken for granted to be true by the church fathers, is nowhere stated outright in Scripture. There are Scriptural assertions of God's immutability, however, and these combined with reigning classical philosophical tenets about to on (“the thing that is,” i.e., Being itself) led the fathers to adopt uncritically the belief in the impassibility of ho ôn (“the One who is”).
(A note on impassibility and impassivity. Because God could not suffer, some [certainly the pagan philosophers who viewed God as impersonal] took the next logical step and maintained that God was impassive as well—that is, that he did not feel at all. Some Christian fathers agreed with this to a great extent and called biblical expressions of God's emotional state [e.g., wrath] to be sheerly anthropomorphic; others held back and allowed that God did feel, albeit not in a way that is essentially tied to suffering like humans do.)
The Christological problem is that God cannot suffer (the fathers believed) but Christ, who is homoousios (i.e., of the same essence) with the Father, somehow did. This is related to a soteriological problem: there had to be a genuine union of impassible divinity and passible humanity in Christ so that we passible humans could be saved from our passibility by union with him and thus become immortal.
It is fashionable today to dismiss God's impassibility as a patristic hang-up, because it is also fashionable to blame everything doctrinal one doesn't like on the Greeks. The massive awareness of suffering in the 20th century (at least to Westerners—it probably wasn't new to the rest of the world) made the idea of a suffering God attractive. And to top it all off God's impassibility as such has scant biblical warrant; in fact, texts like the book of Hosea appear to describe God's suffering forcefully. Jettisoning God's impassibility has a lot to commend it, and it neatly resolves the Christological problem by removing one horn of the dilemma.
But the soteriological problem remains and in fact becomes an eschatological problem. Because if God is by nature passible then so we always will be. That would imply that the absence of suffering in the new creation is not because of the new nature of the redeemed but because of an arrangement of circumstances to take away what might hurt us. So its glory would lie not in making humans stronger but in making the world weaker. While this might be akin to the pre-fall creation, it is not worthy of the new one, which is to be greater. It also reintroduces a Christological problem through the back door, not the problem of Christ's post-incarnational state but his post-resurrectional one: if impassibility is not an inherent quality in God to be partaken of by humanity in Christ, then how was the resurrected Christ unable to suffer even during his forty days with the disciples in this age before his ascension to the Father's side?
Squaring God's impassibility with the biblical witness becomes a bit easier if we are careful to define what suffering is. For humans at least—and apparently for animal life as well—suffering is both pain and weakness. This is true biologically anyway; when I break my arm I feel pain and that arm does not work. This is even true in small ways; when I bruise myself tissue gets inflamed and cells die, bringing a minor incapacity to that part of my body. But it appears that for God pain and weakness are not joined together. God absorbs blows that he genuinely feels and genuinely hurt but make him no less than he was before receiving them. The Holy Spirit may be "grieved" (Eph. 4:30), an emotional pain, but he is no less powerful or perfect as a result of that grief; it does not stop him or slow him down. And this is what must be retained in the doctrine of God's impassibility—not that God does not endure blows or experience pain but that God does not become any the less God, any the less infinitely perfect and infinitely dominant over his creation, as a result. It is God's power, not God's feelings, that must remain unaffected.
This may resolve the Christological dilemma. As God in the flesh, fully God and fully man, Christ truly suffered as both, but the single experience of suffering was not experienced singly by his two natures. In both natures he endured blows and in both natures he felt pain, but only in his human nature did that pain include weakness to the point of death, because only human nature, not divine, can be weak. Thus there can truly be a hypostatic union of natures (i.e., two natures united in the single individual who is the Son) in which the divine Lord of glory himself is truly the one crucified (1 Cor. 2:8), and he feels it and knows it as pain, but at the same time Christ's divine nature remains unweakened and God's impassibility, rightly understood, is unthreatened.
Admittedly, something inexplicable happens in Christ's resurrection. He possesses a "spiritual body" (1 Cor. 15:44), in which the impassibility of the divine nature has been fused to human nature in such a way as to render the latter impassible as well. Thus Christ could still have holes in his body but be even mightier physically than he was before he received them.
Perhaps I need to reconsider my earlier statement of the eschatological problem of making God passible. Because pain, not merely weakness, is absent from the new creation (Rev. 21:4). Perhaps the nature of the new creation is both the strengthening of humanity with the unmitigated power of God to banish weakness and the removal of those circumstances that cause pain (namely sin and all its concomitants, notably death).
Another thing that perhaps must be qualified is the nature of God's power. For a while I have maintained that God's almighty power is not intrinsic to his nature in and of itself but rather is inherent in his relationship to his creation. God has almighty power over everything because everything came into being by him; if nothing existed besides God then "powerful" is a vacuous descriptor of him because "power" implies "power over" something else. Therefore Christ could remain fully God and yet experience weakness as man because his relationship to the creation was altered in his incarnate life, having entered time and being united with the stuff of creation. This has been the root of my explanation of how Christ suffered, how Christ could not know certain things, how Christ could truly experience what we experience, how he could truly be an example for us to emulate, and how he could empty himself without making himself any the less God (an impossibility). But if God's impassibility even in the flesh means preserving his power (see above), then I need to distinguish more carefully what "power" means in this context. God's impassibility in Christ means not that Christ's "power over" creation is unaffected, because that had already been emptied and his miracles were exercised by the Holy Spirit through him. Rather it means (if this statement has any meaning) that his power to be who he is is unaffected, his integrity of nature is unaffected—his perfection, not his dominance. The strength that he retained in the incarnation was not his strength over creation but his strength to persist as God within the creation undiminished by the forces of creation even as his human flesh failed.