AZ Adventures is attempting to create a "Dawn of Civilization" mood and milieu. The hardest part about creating a Dawn of Civ campaign environment is dealing with religion. (Secondarily, magic, but primarily, religion.)
We have a terribly hard time relating to the primitive religious mind, because it was not just a set of different beliefs, those beliefs were embedded in an entirely different worldview. The Gygaxian cosmology is an expression of a modern worldview and a modern understanding of religion. It is totally inappropriate for modeling the primitive theology.
As I have already discussed, the primitive worldview was a "closed cosmology", with the earth at the middle of a cosmic sphere. That part is easy enough to understand. Maybe hard to adjust to, but easy to understand. The really hard part is abandoning the idea of personal gods located on other planes. This is especially difficult because it strikes to the heart of our understanding of the cleric class.
AZ adventures primitive theology is based on the idea of simple dualism: a material plane and a spiritual plane. The spiritual plane is where life force resides. Life force permeates the material world, waiting to break forth in spiritual power. Most spiritual force is most obviously expressed through the living things of the world, such as plants and animals.
However, spirits can potentially inhabit any material body, thus we have the spirit of the wind, the sun, the forest, the mountain, the earth, the lake, and so on. In game terms, these are the elementals. The primitive mind would consider them gods of nature, and druids/shamans attain their spiritual power directly from this source.
Spirits and the Undead
When we die, our spiritual force is separated from the body. Some spirits travel onward to a different location in the spirit world. Many simply fade away, either right away or over time. Other spirits remain to haunt the material world. These spirits are of two types: blessed spirits or hungry ghosts.
Blessed spirits are those who are kept well-fed with positive spiritual energy, given to them by the living in the form of thanksgiving, praise, remembrance, and, best of all, ritual sacrifice.
Hungry ghosts are spirits who are starving for spiritual energy, so they seek to take it by force from the living. In game terms, these are the undead.
Notice how many gods demand sacrifice of blood, which is a gift of powerful spiritual energy. This is not different, in spiritual terms, from how a vampire feeds. Burnt sacrifices are a similar transfer of life force, either the spiritual power of the living creature burnt in sacrifice, or the spiritual energy given to a sacrificed object by its human creator.
The Nature of Gods
Spirits with lots of spiritual energy have the ability to affect things in the material world, from minor effects to miracles. A spirit that exhibits concern for the living will get greater prayer and more sacrifice, and as a result, will accumulate greater spiritual power. This virtuous cycle can result in the spirit eventually being worshipped as a god. Many of the "gods" of a people were once their living heroes whose resting place became a location of pilgrimage.
However, spirits, even ones that rise to the rank of powerful gods, are not infinite, nor omniscient. They are powerful spirit people, but they are located in specific locations, usually connected with their physical remains. Thus, spiritual power can be found at their grave site, or even in their physical remains (especially their bones, which often become handled as sacred relics).
Gods, as spirit beings tied to a specific location, cannot know what is going on in far distant locations, but can only be made aware by what others tell them. Rarely, the spiritual power of a god can be mobile. For example, its sacred relics could be carried around in an ark. Or the god's sacred throne, or representative statue, could be moved to a new location.
The Location of Spiritual Power
Thus, spiritual power comes from interaction with specific locations of power in the world, not from praying to disembodied spirits on other planes. Holy objects are made spiritually powerful by their physical proximity to the the spiritual power of the god. Thus, the dirt around the courtyard, the water in the sacred pool, the chalice used to receive the sacrificial blood, these are supercharged with divine power.
This helps explain why the physical holy symbol is considered the material component of "clerical spells": through its special construction and consecration, it was charged with spiritual power.
Implications for Clerics
Clerics themselves are defined as a class by their close connection to the spirit world. Their WIS is a measure of their natural connection with the spirit world. Unlike magic users, they are not adept at harnessing the natural magic of the material world. Instead, cleric spells are characterized by their effect on the underlying spiritual nature of the material world.
By harnessing their natural connection to the spirit world with disciplined study and devotion, clerics becomes powerful manipulators of spirit energy in their own right, living manifestations of their deity, possessed by the god's spirit and will, and requiring periodic recontact with their spiritual source for recharging. This also helps explain why clerics spread the worship of their god, as the power gained by their god is essentially power gained by them personally.