You could play a triadic knight or something similar (paladin devoted to multiple gods simultaneously) Or a Paladin empowered by an unknown patron, but the latter seems like it would be extremely niche and unlikely circumstance.

What you *don't* have in the realms is people just swearing an oath and becoming empowered from the mere act of swearing said oath. Because, as has been discussed-divine power comes from the gods, and additionally the magic spells of divine casters are formed from weave magic, so you have to have 'the gift' in the first place to cast spells (and those with the particular attunement to be able to cast divine spells are particularly rare)

Fun fact: most 'priests' in the Forgotten Realms can't actually cast spells, because the act of merely being particularly devout isn't enough to be able to channel spells. Neither is it enough to have the particular strain of 'the gift' that allows you to channel divine power because without a divine patron you might as well try to become a wizard.

Thus you really don't have much in the way of 'home grown' divine casters. The setting mechanics just really doesn't support it. Either such individuals are gonna be screened out by clerics of local temples looking for recruits with potential, or they'll get a vison or somesuch and be more directly called by a divine patron that sees potential and a kindred spirit in them. If neither case, you just don't have a divine caster.