Having read this thread, here's my take on a disciple (I have one in my campaign as a player character).
Greg wrote about making everything you do an act of worship, of embodying your god. Plowing the field, making love, baking bread... you can do all of this while contemplating and emulating the sacred stories of your god. A communal worshipper can do this for all the relevant gods in the pantheon (it's possible to have a high ability in communal worship remember), while an initiate and especially a devotee focuses on identifying with his/her deity (or Hero).
A disciple is one who has succeeded. In my view the life of a disciple is a never ending heroquest (mostly in the mundane world though). Every situation he/she encounters, and every decision he/she takes is done as the deity. A disciple of humakt lives the life of a sword saint of oriental legend. As a holy person (and a very dangerous one) people will provide him/her with food and a place to sleep, but the disciple will usually pass through communities without belonging to any of them personally. These people bring the gods to walk among the people.
As the disciples are still mortals, instead of deities, their identification isn't perfect or complete (if it was, it would be like learning the great secret of Orlanth, and becoming one with him). They still make choises as human beings occasionally, and feel human passions (especially those that are appropriate to the deity), but their freedom of action and even thought is severy limited.
If you guys are interested, I can tell how I've solved the problem of having a Disciple of Humakt as a player hero. It wasn't easy :)
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