From a gamemaster's point of view, religion in Trollslayers is essentially agnostic: the gods may, or may not, exist. Most believe, of course, but none in this lifetime may know for certain.
Most gods are defined by one or two (usually intertwined) major facets of daily life (i.e. sky/weather, sun/fire, earth, love/desire, death, etc) as well as a few lesser themes that tie thematically to the greater whole. For example, Aurtan is not only the god of the sun and fire, but also inspiration, madness, creation, destruction, and metalworking. Most gods that humans worship are not, from an outside perspective, either 'good' or 'evil', but rather are more dualistic in nature - the sky god Gwyntcalon gives both gentle rains and lethal thunderstorms. Of course, there may be varying approaches to worshiping a god within splinter branches of the same faith. While many priests of Basla, goddess of death, abhor and fight against the undead, other priests of that goddess study and actively use necromancy, working with and using the undead, perhaps even striving to some day become undead themselves. As in our own world, some of the most bitter religious feuds are not between different faiths, but instead between different factions of the same faith.
In game terms, most priests are Experts, and being a priest is their Profession. The gods, if they do exist, do not grant their acolytes any special abilities; priests must learn any spells (assuming they are capable of doing so) as would any Expert. As with other Experts, any spell that falls within the domain of their faith (a light spell for Aurtan, or a weather spell for Gwyntcalon, for example) can be purchased/learned at the usual cost, otherwise it is doubled. A priest's area of faith also defines their Profession. Priests of Eilide, the earth/nature goddess, would gain the usual +1 bonus to any skill rolls involving tracking, herb identification, dealing with animals, etc. Also, most priesthoods will also gain that bonus when dealing with others, dependent upon that religion's role in society. Priests of a faith that is the local state religion might also be adept in dealing with bureaucracy and legal matters, while the followers of an outlawed religion might be more skilled at subterfuge and stealth.