I found a lot of this book formidably dense. Recently I read an introductory book on Jung by psychoanalyst Anthony Storr that sheds some light, even though Storr never specifically mentions AION. Storr observes a tendency in Jung's thinking to describe the psyche as a self-regulating mechanism, like the human endocrine system. For example, extraverted activity in the unconscious compensates for introverted activity in the conscious (or vice versa). Also, a neurosis may be the unconscious's way of compensating for overly one-sided thinking in the conscious. Similarly, a schizophrenic delusion may be the psyche's (unsuccessful) attempt to restore a lost mental balance.
Examples of this balance/compensation principle in AION:
(1) The Christ symbol. It's a symbol of the Self (like most of the symbols and archetypes discussed in the book), but it lacks a Shadow or inferior component; consequently, the early Christians were compelled to generate the Anti-Christ symbol. However, since the Christ and Anti-Christ are separate entities in traditional Christian thinking, the Western worldview has become highly dualistic and Manichaean, good vs. evil.
(2) The God archetype. As Western thinking has become increasingly secular over the centuries, the God-image has become repressed into the unconscious, where it emerges in savage political forms such as fascism, a worship of the State. (Jung wrote this a few years after World War II.)
(3) Leviathan and Behemoth. "God's monstrous antagonist produces a double because the God-image is incomplete..." (pg. 120).
(4) Sons of God in Catharist legend: Satanael the elder son, Christ the younger son. Similar to the Christ/Anti-Christ dichotomy.
(5) The "higher" and "lower" Adam figures in some Gnostic legends. The higher Adam represents higher states of consciousness; the lower Adam, the unconscious.
(6) The two thieves crucified with Christ. One is destined for heaven (higher consciousness), the other for a warmer climate (unconscious).
Of course, there's more to the book than this equilibrium-of-the-Self aspect. But that aspect ties in with the main theme, the process of individuation (or ascending to a higher state of consciousness) in the Western mind.
Jung really assaults the reader here with his encyclopedic knowledge of religion and alchemy. A lot of his later work deals with esoteric subjects (alchemy, gnosticism, hermeticism, kabbalah). I found a few of the religious subjects, like the medieval "Holy Ghost" movement, to be pretty interesting in themselves, but unfortunately Jung discusses only those elements that relate to his psychological theories.