In Ascetical Homilies III/11, Isaac addresses the nature of the knowledge of God as it existed over the course of three ages. In the first age, there was no knowledge or remembrance of God at all. In the second age, presumably the one of the election of the people of Israel but prior to the coming of Christ, there was a knowledge of God but Isaac calls it childish. Our understanding post-Christ, however, is more mature, better, higher:
We however are renewed in our minds by a new knowledge which was not revealed [to previous generations]. That is why we understand now the Nature which has no beginning, nor limit, whereas those [previous generations] still had a childish thinking with regards to God, believing about Him that He is strict, that He is vengeful, that He repays, that He is just in repaying, that He is wrathful, that He becomes angry, that He remembers the sins of the parents in dealing with their children's children.
For we have a better understanding about God and a higher knowledge of Him: we know Him as One who forgives, Who is good, Who is humble; Who for a single good thing [in us, even] only in thought or even for mere compunction of heart, forgives the sins of [many] years. And not only does He not remember another's sins, but His mercy does away with the multitude of sins even of those who have perished in sin and have already died (III/11, 4-5).
The difference between childish thinking about God and mature thinking about God, for Isaac, concerns the nature of God's disposition towards those with whom he has his dealings: the childish and immature suppose that God is not fundamentally committed to the well-being of his creatures in everything, whereas the mature do.
In Ascetical Homilies II/39, we find even more impassioned language on this topic:
The act of imagining that wrath, fury, jealousy, or other such things have anything to do with the divine Nature fills us with horror, because no one who has a sound mind and intelligence can come to such an insanity as to think such things about God. We cannot even say that He behaves Himself this way so as to pay back evil, even if at first glance the Scriptures appear to say this. Even merely to think such a thing about God and to say that He pays back evil is an abomination. To suppose that He uses so weighty and grave a thing [as Gehenna] as a payback [for evil] means attributing weakness to the divine Nature, because such a thing we believe cannot be found even in people who lead a virtuous and upright life, and who think in their minds in a godly way (II/39, 2).
He goes on to argue that God clearly shows his good will towards sinners by the fact that he preserves them in being and continues to care for them even after they have sinned. He considers the objection: If someone were to say that here on earth God wanted to show his patience towards [sinners] merely so that he could punish them without mercy on the other side, through his childish thinking this person utters against God an unspeakable blasphemy; he undoes His meekness, goodness, and mercy, for which He truly has patience with sinners and the wicked, and he makes Him a slave of passions, as if God didn't permit that they be punished here because His short patience here was preparing for them an even greater evil on the other side. Someone like this not only does not worship God; he calumniates him (II/39, 2).
Now it is of course obvious that even in the post-Christ texts of the Bible, we hear and read talk of God's wrath, of judgment, of punishment, and the rest. Likewise it is clear that in the Old Testament we find numerous affirmations of divine mercy, of God's goodness, of his willingness to forgive sins, and the rest. Isaac has read the Old and New Testaments and he is aware of all that. I don't think his purpose is to propose a kind of quasi-Marcionite distinction between the supposed "God of the Old Testament" and the "God of the New Testament," as if they were different gods.
Rather I think Isaac is suggesting a kind of reorienation of God's moral values or commitments. In light of the most compelling revelation of all -- God's sacrifice in Jesus Christ's crucifixion -- we become aware that deep down, at the heart of every one of God's dealings with us, is love. Isaac's point is not that God never brings affliction upon people; his point is that in doing so, it would be wrong to understand God as really wrathful, as opposed to merely apparently so. (Isaac, as I've pointed out in previous postings, has the typically monkish high-view of divine providence: everything that happens is approved by God in his inscrutable wisdom. He would not suggest that God had "nothing to do" with the various calamities of the Old Testament, for example.) In previous generations persons might have that the calamities which have befallen them are really products of God's anger tout court; in light of Christ's sacrifice for us, we know that what appear to be angry moments with God are really just more difficult or aggressive manifestations of the same immutable love and goodwill with which he always operates.This is true even of hell, which pertains to those who've died, and which Isaac sees as a means of God of purifying sinners.
Mature knowledge of God, then, for Isaac, is that which understands that love and goodwill are behind everything that God does, no matter the appearances or language used to describe them. "Wrath" may be the word used, but the reality behind the appearance of wrath is love. This is not Marcionism since it doesn't deny that God may have done the various things ascribed to him in various Old (or New) Testament texts; it merely calls for a reminder of Christ's cross, which tells us that God is willing even to die for us so that we may know he loves us. Mature theological knowledge insists on divine benevolence and love in every piece of the history of the world, even when it is beyond our understanding (as it may have been beyond the understanding, say, of the Jews during the siege of Jerusalem to understand what was happening to them as God's love).