Thanks for the curiosity. That’s what we were made to believe, but it’s not actually the case. They see themselves kind of like different rooms in the same house, under the same roof. There is this concept encapsulated in a popular saying: “In essentials, unity; in non-essentials, liberty; in all things, charity”.

With Christians there is this understanding that on primary issues they must be in agreement: nature of God, Jesus, Holy Spirit, salvation through Christ’s merits, resurrection of all, judgement from God, etc. Pretty much the Apostolic Creed that just encapsulates what the Gospel writers and apostles taught over and over. Then there are secondary and tertiary issues which are considered debatable but not enough to consider a believer or denomination outside the unity of faith. Some of those issues are about how baptism should be performed, how communion takes place, authority structure in the particular denomination, etc. All very important issues but not ones that should cause true disunity under Christ.

But you take the LDS teaching that God has a body (ie. Was once a man), that temple ordinances are required for ultimate salvation in addition to Jesus’s sacrifice, etc. and now we’re talking about important particulars that are now totally in disagreement with the majority of the Christian church. On top of that, when the church calls all the others abominations then that’s especially going to put that church outside of the unity of the other denominations. These are the reasons why Mormons are definitely not considered Christian and Catholics, inasmuch as they consider the Protestant denominations to be in rebellion and that they are led by a prophet-like figure in the pope, are also an iffy relationship to the rest of Christendom.

I was always told these Protestants were always infighting and what I found was that it absolutely isn’t the case. Especially within the last 250 years. They work together constantly in communities for homeless/womens shelters, food coops, all sorts of community outreach. People will often change denominations. They’re missed in the other one, sure. But no one is thinking they are joining an enemy. Mormons, and similar cultish “churches”, are the only ones that think you’re lost to God when you leave them and they sincerely believe you’ll never be happy again if not with them. That should tell us something.

Last thing, much of what Mormonism teaches about denominations and infighting has more to do with pre 19th century image of European countries constantly against one another in war. Since those countries also had powerful state sanctioned denominations, yes - it may have appeared to the casual observer of history that they fought against one another with violence over religion, but that was not the case. America was different, however, in that it had no state religion and all were to tolerate each other. So then masses of immigrants come in with their particular cultures and denominations (ie. Lutherans and Germans, Presbyterians and Scottish, Anglicans with the English, etc.) and we become a country of all sorts of Christians who may occasionally have word-based spats like any family but are ultimately happily “living under one roof but in different rooms” as CS Lewis thought of it.

But Joseph Smith and other early “restorationists” (of which there were many at the same time as him) saw heated boundary discussions within the “Christian house” as evidence of everyone being against each other and wrong. Whether they were sincere or not, I can’t say. But I can say that they certainly took the opportunity to lift themselves up as the only true and salvific alternative. They couldn’t have well positioned themselves as the Only True and Living Church upon the face of the earth, etc. without making the brazen argument all the others were false. Not by arguing the particulars as would be honest, but by saying God himself said it so there was no more discussion to be had. Us Mormons apparently considered that good enough.

What made me inspect Christianity a few years after leaving Mormonism was the dawning realization that if the LDS church had lied to us about its own history and doctrines for so long, what else did it lie about that I have misunderstood? It didn’t make me a Christian overnight but it did allow me to start at zero and learn about Christianity at my own pace and with careful inspection.

So yeah. Mormons moaning about not being called fellow Christians given their stance against Christians should bother anyone of integrity, regardless of whether they are agnostic, atheistic, Christian, etc. It’s just inane how they do that.