Favour is how much a deity likes a character. All priests receive extra powers as they rise in level, but favour represents other factors, such as devotion to the deities aims, and how much the deity likes the priest as a person. Non-priests can also receive favour, although at a lower rate.
Favour of less than 0 indicates active dislike. 1 to 50 indicates small regard. 51 to 75 is the typical value for followers. 76 to 100 indicates a devout follower. 100 or over indicates the deity likes the person well enough to make them a priest or paladin, although this only indicates the potential to become such. Priests and paladins start with favour 100. Favour may increase without bounds, although over 300 is rare. Such characters are considered especially blessed.
Favour is gained at varying rates, depending on the nature of the deity. Deities of home, consturction or artistry tend not to grant favour for performing typical adventuring tasks. Deities of combat tend to grant considerable favour for such tasks. (Typical favour gain is 10 points per year for performing a role the deity finds pleasing, like being an artist for a deity of artistry. Individual tasks gain about 5 for risky or important tasks the deity finds pleasing).
Favour is used for miracles and acts of divine intervention. The typical devout follower may accumulate enough favour for one small act in their lifetime. Active and devout priests may gain enough favour to be able to call apon their deity to perform astonding deeds. Most deities are somewhat fickle, and may not grant the requested power at that time or at all. (Gaining a spell like power usable once per week may cost 80 favour).
As a general case, appropriate abilities will cost 10 times the Glaive point cost, with no reduction in cost for level delaying. Somewhat appropriate abilities will cost up to 20 times. Abilities which the GM feels the deity wants the character to have may cost as little as 5 times, but this sdhould be unusual. The GM, can, will and should modify these costs, they are a guideline only.
Favour can also be lost for performing deeds or thoughts which are inimical to the deity. A priest of Morgan, deity of healing may lose significant amounts of favour for even thinking of harming the innocent. (Perhaps 20 points). Calling apon the deities power in trivial ways may cause the loss of favour as well, as will casting spells such as divination or commune (generally only 2 point per spell, unless the question is particularly difficult or outside the deities sphere of influence). Asking for an altar to be blessed (for priest spell research say) will cost 40 points. A quest would be not be required.
Priests automatically know their favour with their deity, although in some situations favour gain or loss may be delayed by the DM to hide events. Non-priests can guess their favour, or ask a priest to perform the Discern Favour ritual.
Some examples of favour gain for various religions:
Followers of Hermes gain regard in the eyes of their deity by traveling, visting new cultures, healing, and theft (although not theft for greed or personal gain). Favour is gained by at the rate of 1 per month while traveling, 2 if seeing new areas, or an additional 3 per new culture visited. Healing others (especially by non-magical means) is rewarded by 1 point per week. Theft is 2 points per significant instance, 0 if for personal gain.
Followers of Garal Glitterlode gain favour by practising a craft which Garal finds pleasing. This includes designing gnomish systems, building gnomish systems, or repairing gnomish systems. Favour is gained at the rate of 2/month while performing one of the above, plus 5 per major design seen to completion. (Inventing a bicycle is major, designing a pencil sharpener is not).