That God is eternally the same in everything that pertains to His Nature, that He doesn't change as a result of what happens within the creation, are things that I imagine no rational creature will contradict (Ascetical Homilies II/40, 1).
So opens a discussion on the immutability of the divine nature for Isaac the Syrian. A classical theist right up there with the best of them, Isaac insists on the unchanging and impassible constancy of the divine nature. Nothing that happens within the creation can have any effect on God's divine nature whatsoever. Importantly, these are points that Isaac considers so obvious that no rational creature can deny them; he says that this doctrine of divine immutability is something obvious for all who have a rational intelligence (II/40, 1).
Now when Isaac speaks of the immutability of the divine nature, he is also clearly speaking about the immutability of the divine mental state, if we may put it that way. He makes this evident in the following paragraph, where he speaks of God's attitude towards the demons and sinners:
Neither can we say that the love of the Creator is lesser with respect to those rational creatures which are demons, because they have become demons; that it is less than the fullness of love which He has for those which remain in an angelic state; or that His love for sinners is less than for those appropriately called righteous. Because the divine Nature is not affected by the things which happen, nor by what resists It, and within It nothing happens which finds its origin in creation and which was not in Him from eternity; just as in It there is no love which has its origin in the events which take place in time (II/40, 2).
For Isaac, therefore, the immutability of the divine nature means that there is no change in God's love for his creatures, regardless of what they do. The divine love is not affected by what happens in the created order; it doesn't diminish or increase as a result of human action.
Someone might worry that the divine immutability is compromised by the doctrine of divine judgment. After all, how does God repay sins, if he is not affected by what happens in the world, if his perception of us does not change? Isaac's response is to deny that God repays sins and to insist on the immutability and impassibility of the divine love.
For Isaac, this truth is important for understanding the nature of Gehenna. If the divine love is immutable and impassible, then it would be ridiculous to suppose that the sufferings of those in Gehenna are imposed upon sinners as a payment for their sins, and that God intends that they remain there forever. Isaac denies numerous times that God is involved in any kind of payback for sins; such a thing is foreign to the divine nature. He says in the previous homily (II/39, 2) that to think that God is patient with sinners here because he plans on punishing them harshly for their abuse of his patience on the other side is childish, unspeakable blasphemy, and in the end calumny against God. He will have nothing to do with it.
No, the immutability of God's love means that his act towards the creation, regardless of its recipient, is always motivated by love and is not a calculated response to a person's right or wrong actions. Such are all things which come from [God], even if they may seem otherwise: with God they have nothing to do with payback [or vengeance], because He always looks to the advantage of those with whom he behaves himself thus [that is, in ostensible wrath or punishment] (II/39, 5).
An important evidence for Isaac that God is not concerned with payback or balancing the moral scales is found in the revelation of Christ:
So then, everything which comes from Him and resembles a punishment or condemnation doesn't come so as to make us pay for some past evil deeds, but for our own advantage which we may gain from them, because they make us conscious of the things that are past, only to plant in us a hatred towards sin. . . . If things were not so, what then does the coming of Christ have to do with the actions of the generations prior to it? Might this immense mercy be a repayment for those evil deeds? Tell me, then, if God is the One Who repays [our deeds] and if everything which He does is a repayment, what sort of fitting repayment do you see here, o man? Show me! (II/39, 15-16).
The mercy shown in Christ is anything but a fitting repayment for previous deeds. If God is concerned with justice and if a concern for fairness and attention to merit is what motivates his actions, then Christ could never have come. But if Christ has come, then this tells us that actually God is not concerned to repay according to merit at all. These are mere appearances; the reality is that God is always and ever concerned to do us good, and to have us enjoy life in fellowship with him in holiness. Nothing I can do can change God's intentions for me; nothing you can do will ever change the fact that God wants fellowship with you in eternity.