A Primer on Faith

What does faith mean, in a world where gods are not just real, but highly visible members of the cosmos?
Faith behaves like a currency which isn't so strange in a universe Good and Evil are real, measurable things. Every intelligent, ensouled* creature generates a little faith, a strange intangible thing that fuels gods and their powers. Creatures who follow a specific religion direct their faith to that god, to be spent how the god sees fit. This is very reminiscent of the capitalist idea of "voting with your dollars".
A creature may follow a god for one of three reasons. First, they want their faith to strengthen a god whose principles they believe in. People who value charity and compassion would naturally gravitate towards Mitra, granting her additional strength to help build a more compassionate world. Followers of Gro'tag want a world where martial power (specifically that of Orcs) are uplifted, and help for the weak (especially things like healing spells) are suppressed.
Second, they may want specific power from gods. Zealots of Orcus may not specifically agree with the world Orcus wants to build, but they are happy to worship him in exchange for power. Many a person who felt wronged has gone to Orcus, offering to do bloody deeds in his name in exchange for the power to carry out those deeds. A few of the paladins of Muir are those who want to bring about a powerful, positive change in the world, but lack the strength to do so, leading them to seek out divine power.
Thirdly, devotion to a god plays a big role when a mortal soul finally leaves their dead body and makes their way to Pharasma's line. Nearly every creature has some vague idea about the kind of afterlife they want, and many choose their god on that basis. More than a few petty lawful-evil crime lords believe they'd be a big noise, if only they had the chance. To them, the Nine Hells promises an eternal life of dominating the weak (although it doesn't always work out that way). Some, upon reaching Pharasma recant their faith or have a change of heart and try to find their way to a different peace.
So what of the manifestations of faith among mortal heroes?
Clerics have a deep understanding of themselves and their relationship to a god. Praying really hard and carefully following ceremonies and rites isn't enough. The person often must have an epiphany, a moment of clarity where they truly understand themselves, and how they might form a personal relationship to their god, acting as their mortal agent.
As an example, Marle was a young girl who grew up in Tarren's Junction, and learned about the role that Sarenrae used to play in Valley affairs (specifically, in opposition to Tsathogga). She studied both history and religion, and understood that while neither Tsathogga nor Sarenrae were part of the contemporary Valley, one day the Dawnflower Goddess may be needed again. The knowledge that her purpose was to keep old stories alive to protect people yet to be born was the truth she needed to access divine spellcasting.
Druids work in a very similar way, but sometimes follow a more natural aspect of a god, such as Freya's fertility, Darach Albith's love of Dalish forests, or Tsathogga's home in decaying swamps. By establishing a personal connection to these places, druids access a different set of powers.
As an example, Drusilla was a follower of Freya, and her ideas of both fertility and the hunt. Drusilla's focus was on wolves, a common animal predator in the forest of hope. She made sure their numbers never grew too few or too many, and personally dealt with hazards that posed a serious threat. It was her connection to the wolves and their role in the biosphere that gave her power, rather than her deep understanding of Freya.
Oracles are a weird lot. In their case, at the moment of their birth a god or one of their divine agents has imparted a mission or purpose unto the child. The child doesn't have any choice in the matter, and often their life and purpose is planned out for them before they even know their own name. The vast majority of oracles have no problem with it; gods have ways of knowing a child's future personality and nature, and are careful to only select those who'd be on board with the idea. Exactly why Oracles are selected varies from god to god, and is based on what aspects of the future they can predict with reasonable certainty. Since the loss of prophesy that came with the end of the 4th age, the number of oracles has reduced dramatically.
The easiest example here is GDECs own Tabitha Wolfsbane. Tabitha was selected by Balthazar for her powerful personality and drive. She would not normally have been a follower of Balthazar, as he was relatively unknown. Tabitha succeeded in her mission almost to a fault, Balthazar had underestimated the power of her convictions.
Last, but not least are the Paladins, Zealots, Blackguards, Grey Knights, and all the other champions of the divine and profane. These people do their best to act as exemplars of their god, shining ideals for all to see and follow. These people often have the strongest faith, as their self-confidence and belief in their god are one and the same. They are relatively few in number, as they require both a strong personality, and the martial prowess to bring their will to bear. 
In the case of Lady Kingsley (blessed be), she wanted to be a paladin, but lacked the conviction and purpose that drove those holy knights. It was only after a chance meeting the ghost of Saint Izolda that Kingsley was able to find a path, and learned to follow in Saint Izolda's footsteps.
There's a little more to cover - champions, chosen and favored, but that can wait for another time.
*Generally this means creatures with an intelligence score 7 or higher that's animated by positive or negative energy, as opposed to creatures animated by magic or clockwork, such as most golems.