It is not my opinion that the magic of clerics comes from an external source.
No, but it appears to be your opinion that clerical magic is not associated with their person. But. Paladin magic is the same magic, from the same source, and explicitly, "[draws] on divine magic through meditation and prayer to cast spells as a cleric does], yet uses...Charisma as their casting stat, even when explicitly casting spells as a cleric does. They are conduits for divine power external to themselves. Moreover, paladins don't swear an oath to have access to their spells. Their oaths to their gods only give them access to a handful of additional spells, as far as spells go, nothing more, nothing less. Warlocks aren't gaining magic through themselves either. They are explicitly bestowed this magical power by their patron, hence it also comes from an external source.
I understand that the English language is dynamic and accommodating, but the game leverages its own definitions.
Evolving and changing definitions, and definitions that I am disputing. Hence stating that wisdom shouldn't, in my opinion, be a stat at all. Because the D&D definition is very different indeed from the actual definition of the term in use.
Hence, part of the very reason for the disconnect people are expressing in this thread and others, in no small part. If a game is going to use terms, and those terms begin to stray too far from the real-world definitions of those terms (and they aren't highly specialized terms like mana, or akasha, or quintessence, or similar expressions for magical energy, for example), while attempting to continue to use them in a manner like the real-world definition, then it is perfectly fine to question the very premise of using those terms to begin with, as being replaceable with more apropos terms. If Intelligence were evolved after a few iterations of D&D editions to state that Intelligence covers your ability to sense things better, to see, to hear, to feel, to taste, to smell things better, I would similarly dispute the very notion that it is being used in such a manner. As it is, for all intents and purposes, from a mechanical perspective wisdom is actually trying to do exactly that; if you are wiser, you are automatically more perceptive of your actual surroundings than anyone else, all else being equal. That is, in my opinion, ludicrous on its face. It is worthwhile to point out and discuss such perceived ludicrousness. Doesn't actually require one to talk down to someone else, and use the circular logic of, "this authority says so, so it's so, period, end of discussion". Which is precisely what it seems you are trying to do; end the discussion for others.
In the game of Dungeons & Dragons, clerics function in a clearly defined way.
Neither an oath or barter are required.
Neither an oath or barter are required in a mechanical sense, and a mechanical sense alone. You don't actually know whether or not an oath is required of any given god within the lore of even D&D.
But in a strictly mechanical sense, and a strictly mechanical sense alone, are no oaths required. That does not, in fact, mean that a given god doesn't require an oath from one of their clerics to grant them the ability to cast spells. There are a great many off-camera requirements that have no mechanical requirements. There is no mechanical basis for being literate to read a language in D&D, either. Look at the DMG and PHB both. According to the rules and mechanics of the game, literacy, the ability to READ AND WRITE languages isn't mentioned at all. Thus, is it safe to assert that in D&D, if you learn to speak a language, you automatically know how to read it as well? It it reasonable to say that the moment you can speak a language, you can read and write in that language automatically, because reasons? Or do you think that perhaps, just perhaps, off-camera there is an additional, non-mechanical requirement that you need to be taught to read to know how to read? A requirement that isn't explicitly stated, but rationally is implicitly present nonetheless.