What is the theme in A History of God: The 4000-year Quest of Judaism, Christianity, and Islam by Karen Armstrong?

Mysticism is another theme in this book. Mysticism, or the experiencing of the godhead directly by an individual embarking on the "internal journey," has been a common but resisted idea in all three monotheistic religions. In the Western churches, mysticism was never part of the main stream of religious development and was marginalized and outlawed. It would seem that the ability of an individual to communicate directly with the Supreme Being, without going through the intermediation of priests and prophets, represents the danger of a loss of control on the part of these self-elevated officials.