I’ve been tinkering with an idea for a while, a way to divide the Cleric spell list from the M-U spell list (because there’s some thematic overlap). All Cleric spells are non-visible.
For example, instead of immediately healing damage with a Cure Light Wounds spell (stopping bleeding, restoring lost blood, setting and knitting broken bones), nothing visible happens. Either the Cleric amps up your natural healing ability and you get more healing by resting tonight, or the wounds look just as bad but you John McClane through the pain and stand up to fight. In the first example, the healing still feels miraculous but it’s delayed and the Cleric’s magic is less powerful. In the second example, we’re explicitly on board with explaining HP as stamina / willpower / fighting skill / morale.
In both cases a skeptical observer could argue some non-magical explanation. Maybe his recovery outcomes were just way better, or the wound wasn’t as bad after all. Maybe the Cleric has just given him a really good pep talk. Either way, a Cleric’s abilities aren’t obvious enough to be proven, and by extension his claim to receive them from a deity. Let’s ignore the possibility he could be deluded or lying and receive his spells from some mortal source.
I’m also assuming the players around the table will always be able to spot mechanical effects. You might be able to put everything behind the DM screen but that would be much more difficult. But if your goal is to make the players wonder whether Jimmy is actually playing a Cleric it can be done for a little while but again it’s tough.
Another example is Animal Friendship. Sure it’s weird that he can get a wild animal to follow him around and clean his dishes, but maybe he’s just an animal trainer!
This unfortunately prevents us from having a Flame Blade spell. But the primary purpose of the spell is to give a melee attack, which is better against Undead, with a secondary purpose of having an instant magical fire which can ignite objects and shed light. The primary spell effect could be replicated with an unarmed damage bonus. A skeptic would say maybe the Cleric just smacked the guy really hard or in the right place, or the guy was intimidated by being attacked by a Cleric and went down easier.
We can replicate the secondary benefits of the Flame Blade and also the Light spell if the Cleric has a spell to make lighting a fire extremely easy (which could be worth a 1st level spell slot in a wet environment the same way Create Water is worth praying for in a dry environment). If he casts it on an oil lamp, torch, etc. the fuel also lasts longer (which gives us a reference to Judaism too!) which makes the spell worthwhile in low-fuel situations too.
Instead of Create Water, the Cleric is able to find nearby fresh water. Same with Create Food and Water – maybe this guy is just really good at foraging!
Hold Person is problematic as written. But it’s also an extremely powerful combat spell that can safely be canned because Clerics are already really strong characters. Or give a weaker version that halts people while the Cleric harangues them and stares them down impressively – but if they are attacked they’re no longer affected. Because the effect is weaker you could increase the number affected to compensate.
Instead of Invisibility or Silence, give enemies a lower chance to notice the party. How many times has a guard suddenly realized a small animal has wandered into his area and he didn’t notice? Or a fellow guard? Embarrassing but explainable.
Instead of outlining enemies in Faerie Fire, enhance the vision of a friend so he sees an extra reflected glint or just feels where the enemy is, and can attack accurately.
1st and 2nd level spells are so easily reskinned so nobody knows if the Cleric is legit. Typical village priests will still need to work to convince laypeople instead of impressing them by flaunting amazing magic spells.
There should also be a spell as a reversal of the intent of Locate Animals or Plants, which helps reduce random encounters.
Only once we get into 3rd level magic do we need to make things truly visible. By the time we get to Raise Dead it becomes difficult to keep things ambiguous. Then again, according to the 1E DMG 1st and 2nd level spells are cast through the Cleric’s own study and influence, 3rd and 4th are granted by lesser divine intermediaries, and 5th-7th are granted by the deity itself.
I love the perspective this puts on the spell lists. Cleric levels 5 and 9 are super important because they represent gaining a connection to powerful servants of their deity or the deity itself respectively.
At level 5 you can cure diseases / blindness / deafness / curses / paralysis, create food, animate skeletons and zombies, speak with the dead, walk on water and flames, and call down lightning bolts from storm clouds.
At level 9 you can speak directly with your deity, raise the dead, walk through the air, travel to a different plane of existence, and call down pillars and walls of flame.
So there’s really a limit on how far you can go making Cleric powers ambiguous in roleplaying. But I think it’s worth it enough to make the effort to reskin 1st and 2nd level spells.