There's a legend about the Battle of Brunanburh, at which King Aethelstan confronted the allied kings of Scotland, Dublin, and Strathclyde in 937 AD. Before the battle, one of Aethelstan's soldiers was lying sick: he happened to be a Devonshire man, so in his sickness he prayed to his local saint, the martyr St Nectan, to heal him. That night he had a vision of St Nectan, and in the morning the sickness was gone and he was well enough to fight.
The interesting bit isn't the healing: it's what happened next. The Devon man told everyone about his miraculous recovery, and soon word reached King Aethelstan. Aethelstan was understandably nervous about the coming battle, so he asked the soldier which saint it was who had proven so receptive to his prayers. The soldier told him about Saint Nectan, and assured him that Nectan was always swift to intercede on behalf of those who had faith in him. Aethelstan prayed to St Nectan, won the battle, and was a generous donor to the cult of the saint thereafter.
Just think for a moment about what this sequence of events implies about Aethelstan's mindset. He's king of a sparsely-populated nation made up of hundreds of scattered communities, each with their own local shrines and saints and martyrs and holy wells, most of which have never been heard of by anyone outside their local area. He takes it as given that these saints are capable of interceding with God on his behalf, and that some of them are more likely to do so than others, but crucially he doesn't know which ones. The obscure hermit-saint revered in some provincial village might turn out to be exactly the guy you need to pray to in order to resolve a major national crisis. Working out who to pray to under which circumstances isn't a matter of set dogma, established long ago and handed down by recognised authorities: instead, it's a work in progress, to be figured out by trial and error. Building up a working knowledge of all your national saints, and cultivating suitable relations with their respective cults, becomes a potentially important element of kingship.
This is a mindset that would, I think, have made intuitive sense to the Tengriist and Shamanistic cultures in the history of Central Asia, for whom the question 'which spirits have authority here?' was one that every nomadic people had to confront regularly as they roamed from place to place. It would have made sense to the Romans, for whom working out which set of local gods to buy off was an integral part of the process of conquest. It is, however, a mindset that seems to be very rare in D&D and associated games, where most fantasy religions seem to have completely codified understandings of the sacred rather than the more experimental approaches that have historically been so commonplace. This strikes me as a bit of a pity - there's so much more gaming potential in the latter!
Imagine if, instead of being a fully worked-out institutional religion like counter-reformation-era Catholicism, your cleric's faith was something closer to Aethelstan's version of Christianity, a hacky work-in-progress always subject to revision based on the latest discoveries. A huge amount of your adventuring could be motivated simply by the desire to learn more about different local gods or saints or spirits, which you would do by visiting lots of different shrines and making lots of different offerings just to find out which ones work best. In a world where most people stick close to home, worshipping their local gods, an adventuring cleric who's been all over the place could become a real asset simply because of their breadth of spiritual experience. ('Actually, my liege, over the mountains they have a saint that they pray to in exactly this sort of situation...')
Probably the easiest way to represent this mechanically would be to have knowledge of certain spiritual practises grant access to new cleric spells. In most cases these spells might only be available to clerics who'd actually gone to the trouble of visiting whatever remote shrine they are associated with, but sometimes just knowing the name and rituals of the associated god or saint might be enough. Imagine the prestige to be gained in being the cleric who brings such knowledge back to their cult centre and thus unlocks a new spell not just for themselves, but for their entire religion!
And it works for quest-givers, too. Obviously every ruler is going to want to have the latest, sweetest spiritual technology on their side. Obviously they'll want to aggressively investigate rumours of holy sites, obscure shrines, sacred springs, and so on, in the hope of giving themselves and their clergy an edge over their rivals. Everyone knows about all the most famous gods and saints, so they just cancel out: the real advantage is to be gained from being the first one in on a hot new discovery, and they're almost always going to come from way out in the hinterlands, or someone would know about them already. Of course it's going to fall to your party to make the long, dangerous journey through the wilderness to the half-ruined shrine of some obscure local spirit or hermit, so that your cleric can check whether they have enough spiritual mojo to be worth adding to the national liturgy.
Just think of the adventure opportunities!
1d10 local god adventure opportunities
- The king wants to develop a new industry, but because it's not been historically practiced in his kingdom he doesn't know who the appropriate gods/saints/spirits are who oversee these things. Your job is to go to some foreign land where this trade is already established and surreptitiously steal all their knowledge about how best to honour, petition, and placate the relevant spiritual beings, all without giving away the secret of the king's economic plans. You'll get a bonus if you can learn the secret rites their guildsmen carry out behind closed doors!
- In your grandfather's day there was a weird old hermit living in the mountains. Now people are claiming to see him in their dreams and there are rumours of miracles occurring near the remote cave where he once lived. Your job is to get out there, try to work out if he really has become a legitimate god/saint/spirit, and - if he has - find some way to integrate him into the local religion. (Maybe a shrine could be built in his cave? Or maybe you could find his bones and take them to the local temple as holy relics?)
- The king is trying to integrate a recently-conquered frontier region into his kingdom, and he needs someone to do a spiritual survey. Your job is to roam from shrine to shrine among a resentful and rebellious population, cataloguing their local gods/saints/spirits and working out which of them, if any, might be worth adding to the national cult.
- Old chronicles speak of a god/saint/spirit who once bestowed powerful blessings upon their worshippers, but its cult centre has long since fallen into ruin, and no-one remembers the rites by which it was once honoured. Might there be something out there worth salvaging? You'll have to voyage though the wilderness to its abandoned shrine and start making experimental offerings to find out!
- The king has a major project planned and he needs as much sacred mojo as possible. Your job is to visit the shrines of the relevant gods/saints/spirits, obtain their sacred items and holy relics by whatever means necessary, and bring them to the capital to ensure the project's success. Naturally, you can expect the locals to violently resist the removal of their treasures.
- As 5, but from the other side. A bunch of thugs with a royal warrant just rode into your local shrine and carried off the relics of your local god/saint/spirit to the capital, claiming that the king needs them more than you do! Now your community looks to you to steal them back, and to establish a new, secret shrine where they can be safely kept in future.
- The king is planning to hold a major religious festival to bring the blessings of heaven upon his newly-built navy. It's going to cost him a fortune, and he needs to be sure that he's getting the maximum bang for his buck. Your job is to roam the remote storm-wracked islands and pirate-haunted headlands where all the best gods/saints/spirits of the sea seem to have their shrines, and find out which ones are most worth honouring in the festivities. Expect every single priest you meet to try to hustle you about this.
- There's been a disaster - but all the priests insist that they've been carrying out their ceremonies perfectly! Clearly some unknown god/saint/spirit is offended - but which one? Your job is to divine which obscure spiritual entity has been neglected, make a pilgrimage to their remote place of power, and make whatever offerings they require in order to slake their wrath before the kingdom suffers even further calamities.
- Sometimes the god/saint/spirit you need to pray to has their holiest shrine in a really inconvenient place, like the other side of a monster-haunted wilderness or the middle of an enemy kingdom. Your job is to undertake the perilous journey there to make offerings on behalf of your king, so that he can win their favour for his latest scheme.
- Out in the wilderness the remains of some forgotten shrine to a god/saint/spirit have been discovered, but no-one recognises the names carved on its ancient stone. Is it holy or unholy? Does power still reside here, and if so, is there enough of it to make it worth re-establishing whatever vanished cult once built this place? Best do your research first: trial-and-error offerings may risk causing offence that your kingdom can ill afford...