I’d be interested if anyone here has any evidence that the sources of power for such paladins isn’t still divine in nature, though, even if it comes from the divine portfolios or domains in some cases rather than directly from specific gods. I don’t remember seeing anyone on these forums claiming that, and I had thought paladins powers were always still divine, but as I’ve said before on these forums, I’m a 5e newbie and am learning as I go with BG3 and certainly have neither read every post here nor remembered every one I have!
The lore text before the crunchy stats of Paladin mention swearing your oath before nature spirits and Fey, but to find more info on that you'll actually have to go back to older editions. 5e is continually thin on world lore, which is one of my ongoing complaints with it. 5e also has a bad habit of lumping non-deity powers in with the deities. In fact the entire term "Powers", which was an umbrella terms for beings that were operating at a deity-like level, seems to have vanished.
Faiths and Pantheons from 3rd edition talks about demons and devils finding ways to get and give power from mortals in the same manner as deities, and details some domains, favoured weapons, and portfolios for them. As of 5e we actually have multiple deities now that were once fiends, because of this, but you wouldn't know it from only reading 5e books.
Same with primordials, we have at least 5 that are lumped in with the deities in 5e, and can fulfill all your Clerical or Paladin needs, but again, you wouldn't know they are primordials, just from reading 5e books.
So I can make an Ancient's paladin who swore their oath before a unicorn, who technically gets their powers from Lurue, who is an Archfey (and technically a lesser deity as of 5e).
