That’s an additive change, not a conflicting one.

What you’re saying is like “the US amended its Constitution to give women the right to vote, so you can’t claim they grant the right to free speech.” The fact that the 19th amendment came after the 1st amendment does not mean the 19th amendment disagrees with the 1st amendment; rather, they address two different subjects, both of which independently needed fixing by addressing them in a foundational document.

If anything, the Filioque, as I understand it, strengthens what I’m saying, because it was a change to say that the Holy Spirit proceeds not just from the Father, but from the Father and the Son (Jesus), since they’re one.

That contradicts Mormon teaching even harder, since Mormon teaching is that Jesus is a distinct, separate god, and is not “one” with the Father (Yahweh) in the way that Jesus himself said he was (“I and my Father are one” in John 10; “whoever has seen me has seen the Father” in John 14) or that the rest of the Christian New Testament teach (as distinct from the Mormon editions, which have been changed based on Joseph Smith’s “visions” that all existing manuscripts were somehow secretly corrupted from some hypothetical original text that has never been found nor referenced in any writings prior to Smith coming up with the idea).

If the best you’ve got is “the globally-accepted definition of Christianity that was agreed upon immediately after its founding millennia ago by the closest adherents of Christianity, and was formally codified within the first couple hundred years only so as to explicitly name disagreement with this definition as specifically not being Christianity, and which was only ever disputed in mid-1800s Utah by a random dude who wanted to legitimize his brand-new polygamy cult by pretending it’s part of Christianity is not the definition of Christianity”

…I don’t think you’re in as strong a position as you seem to think.

They believe that Jesus was God Himself (not another god, or a dude who became a god, but the one God Himself) who came down to die so that humanity could, depending on Christus Victor or Penal Substitutionary Atonement, either be saved from the cursing effects of death or else have their sins atoned?

Or do they instead believe that Jesus, an entirely-separate god that the God and some other “Divine Mother” gave birth to, died to pave a path to becoming a god yourself?

I don’t have to. The Christian church formalized it in 325AD. Joey Smith just wasn’t creative enough to come up with a new heresy. He moved too quick to “okay so let’s do multiple wives and also since my wife is objecting to that God says she’s no longer a prophet and is actually a false shepherd” when he should’ve been doing his homework to make his con more credible.

https://en.wikipedia.org/wiki/Nicene_Creed

EDIT: I think maybe you’re confused? I’m not asserting that the uppercase-O Orthodox Church is all of Christianity. I’m using the normal plain meaning of “orthodox”, which is “adhering to the fundamental principles held to be true by a religious group.” All Christians believe that Jesus is part of the Trinity and is fully God, because that’s what Christian means, in the same way that I can assert that all watercraft are things that can travel in water. The LDS faith is not Christian because their cult leader prophet decided stupidly to pick the defining trait of Christianity as a thing he wanted to change.

The Son of God who is fully one with God Himself, the second person of the Trinity, which is one God in three persons.

A ton of the earliest writings in the church were dealing with explaining the Trinity, and nearly every time for the first several hundred years that the Christian church found it important enough to get together and publicly denounce someone as a heretic, it was because of denying some part of that equation (either the Trinity itself, or Jesus as fully God, or Jesus as fully God and one with God the Father). The LDS chucks it out in favor of their own weird “nah, actually everyone’s kinda a god, that’s what’s up with Jesus.” A history dive on “begotten, not made” and the Nicene Creed is helpful here, comparing it with LDS doctrine on the nature of Jesus and on the end times.

That makes them heretics to orthodox Christianity (for denying that Jesus is not “one with the Father”, all homousia) for a similar reason to why Islam views Christians as heretics (because Islam views the Trinity as an assertion of multiple gods, counter to “Allah is one”).

A lot of people see a sort of “X derived from Y” and assume based on lack of any further digging that X and Y are interchangeable, when the very derivation from should call to attention that there was a fork in the road where beliefs diverged. Some of those forks are more divergent from one another than others; Presbyterians and Baptists have more in common than Presbyterians and Catholics, and those two groups themselves have more in common than Catholics and the Eastern Orthodox, and still Catholics and Eastern Orthodox have much more in common than LDS and any Christian denomination you could pick from a hat.

Interestingly, if folks like this read their Bibles – especially the Genesis part they love so much – they’d see that the one of the very first commands God gave to humans in there was to tend the earth as a garden. Like, it’s one of the first things ever said to a human in the whole book.

But like a lot of the Bible, right-wing types tend to ignore that because religion is a tool for control for them, not a guiding morality that constrains their unchecked lust for power.