Gods and "Mana"
In NWolfNES, I say/think that "gods" are ghostly creatures, who are addict to life energy. They consume life energy, where this life energy comes from or why, is not known. Life energy gives gods power to control real world powers (or influence them). Life energy runs in countless numbers of tiny lines and these tiny lines run through everything: You, your mind, forest, trees, insects, wind - everything. Gods can only sustain their life thanks to these lines, but creatures with minds can focus energy and send energy towards gods they pray to. More intelligent and religious prayer = more life energy for god gets. Highly religious cult leader praying would offer as much life force as 30 villagers, who rarely pray do in few days.
By influencing lines with small amount of life energy gods can influence world around them. They can cast mircacles, Like: heal wounds, summon creatures, call rain, create tornadoes and even create fire. To get more faith/prayers/life energy they require followers - priests, acoytes (sp?) and simple believers. To get more believers god needs to prove his strength (by doing miracles and helping in war). Loss of followers means loss of life energy, so gods protect their followers. For an example, losing highly religious priest is comparable to losing a group of 30 civilians, who rarely pray.
Religious events, like sacrifices grant god huge amount of life energy (as priest redirect all freed life energy to god). Sacrificing himself, made by highly religious and devoted priest by his own will, would give off as much energy as 200 simple followers would give within 5 years. And more life force god gets, stronger he/she/it/them become
Mana
Mana is skill of person, that allows person to manipulate mana lines around or in him. Manipulating mana lines is different from gathering mana for a prayer. That means good shaman, who has never prayed in his life, is not much better at religion than a simple commoner. Countless of small lines run through everything (including bodies), a person who learns gathering and releasing life energy that runs through him, can essentially do same things as gods can. But being earthly and non-ghostly limits caster from seeing everything (or more like "feeling"). Not being able (nor used to) to manipulating life energy that flows in life lines, also means that very highly skilled and talented shaman would be roughly equal in strength to a rather weak god.
A person can easly be both highly religious and a good shaman - in this case he can manipulate life lines around him (cast spells) and he can pass huge amount of life energy to god (knowing how to manipulate life lines AROUND you and IN you are different things. Casting spells is different from praying.) In theory one can become a god - first one must learn to manipulate lines around him, then he must learn to consume life energy that is sent towards him via prayers (and get someone to pray towards him). This increases powers of mage quite a bit, but it is hard to make people believe that that person truely is a god. Hence why half-gods prefer to use ghouls(undead), zombies and lowly creatures (gnolls, imps). Despite their "followers" are able to pass only small amounts of life energy via prayers, they are simple minded and easy to convince. Inexperienced person can not use full energy from a prayer either.
If you say, start religious cult that worships polar bear shaped, children loving god. One or more of these godly creatures will surely attempt to pose as that god and help you - while fending off others, who might also attempt to pose as that god too or steal prayers. If god loses all his followers and has taken a shape of will/mind his followers believed him to have, god usually dies (gods take shapes according to how they are imagined, unless god possesses somebody).
It is possible to get mana addict (that is not good).
It is possible share body with godly creature (However, to be the one in control of the body, one has to be very talented shaman or skilled religious person).
It is also possible to get possessed by "demons", "ghosts" and creatures like that. Demons for an example, are similar godly creatures to gods (perhaps even same creatures). These creatures possess body and used it to feed their addiction for life energy. They are quite weak as controlling earthly body consumes much of their energy. Possessed creatures will eventually transform into true demons - shape what demons controlling body preferre. Possessed victims gain strong magical powers and are highly dangerous, it is very rare possessed creatures to be "friendly".
Gods are not shaped before they manage to catch random prayer. Say, if your orc prays reguraly to non existing whale god, one of the gods (who is more lucky) will take form of that god. Once god has taken a form, he/she cannot untake it, so first follower is usually given most protection by god. Of course, it is rather hard to make other peoples to follow you.
Gods prefer to stay near their followers - to ensure their followers safety and to hunt for more energy.