I also, put very simply, consider it a one man form of Channeling (you're just drawing the power from yourself).
Well, by this reasoning, all spellcasting can be classified as a type of channeling; clerics channel power from their god(s), magicians from the environment around them, and mentalists from themselves. Not a bad way to look at it.
Pretty much how I define it right there. Essence you pull from elemental energy, Channeling you pull from your faith (deity or sphere) and Mentalism you pull from yourself.
In the end a deity can originate from any of the sources of magic. Channeling is the easiest path, but Mentalism and Essence have the capacity to get you to godhood if you put enough effort into it before someone stops you. This explains how a mortal could potentially obtain the power to cause a deity real problems or even become one.
Normally extremely powerful Essence users do not move on to godhood (they'd need a very good very large source of power - say the God of Fire using the Elemental Plate of Fire as it's power source), but extremely powerful Mentalism users can eventually figure out how to draw the power out of other sources much in the way they do from themselves. This is where I can explain a "God of Fear". People don't
worship the god in any real way, but the god can feed off of peoples fear (absorb their mental energy). So, the more fear in the world and the closer the 'fearing' individuals are, the more powerful the deity becomes. So, maybe an evil mentalism based humanoid learned to draw from the projection of fear from other individuals, driving him to become more and more evil in order to gain more and more power... and eventually *poof* you have what the world sees as the God of Fear. Then if he starts gaining followers, and has enough power and knowledge to do so, he can start "Channeling" his power to his more important/loyal followers to do his bidding. His followers? Channeling users.
My deity heirarchy is setup so there are a very limited few 'original' gods (which I do not explain where they came from) that will only interact with the physical world via projection or avatars. The rest of the deities were either 'created' by other deities or were mortals that gained enough power to join their ranks. True full on deities would not be able to be completely destroyed (you'd just be destroying a projection or avatar), but most of them could be if you were powerful enough or found the proper method.