Some of the very earliest Christians rejected the Doctrine of the Trinity before your Nicene Creed ever existed. Catholics and Protestants have different beliefs in terms of salvation by faith, predestination, the eucharist and others. You're arbitrarily selecting some beliefs Protestants and Catholics have in common and Mormons don't in order to exclude them. The Venn diagram is three adjoined circles. Catholics and Mormons both believe their leader is God's representative on Earth in a way that Protestants don't, for example.
Christianity is an organic tradition.
You either come from that tradition, perhaps breaking off at some point, or you don't.
You can't make up a new religion and then decide that it is part of someone's else's unless they accept it as well.
But this is a very dull argument. I am not going to change your mind. You are not going to change my mind.
Let's say we give it a rest
You think many founding Mormons were anything other than Christians?
Or founding Christians anything other than Jews?
They were both, just as Mormons are Christians.
Mormons believe they are Christians. Christians do not believe that Mormons are Christians. @Socrates believes that, despite not being either a Christian or a Mormon, he has the right to determine the relationship between two great faiths.
That's all we can really say on the matter.
In respect of your earlier point that the Mormon founder were Christians, that doesn't really make any difference. They didn't take the Christian tradition and amend it by saying "actually we disagree with this bit". They invented something new and cherry picked certain parts of the Christian tradition to provide some substance.
A parallel might make it easier (and I want to stress that I am not trying to identify any party with any religion in the example):
UKIP, I think it is fair to say, grew out of the Conservative Party. There may be a heck of a lot of differences in what they believe, both in policy and in leadership, but they are still part of the conservative tradition.
The BNP might have certain beliefs in common with both UKIP and the Conservatives. But they are not part of either party, and they are not part of the conservative tradition
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That's all we can really say on the matter.
In respect of your earlier point that the Mormon founder were Christians, that doesn't really make any difference. They didn't take the Christian tradition and amend it by saying "actually we disagree with this bit". They invented something new and cherry picked certain parts of the Christian tradition to provide some substance.
A parallel might make it easier (and I want to stress that I am not trying to identify any party with any religion in the example):
UKIP, I think it is fair to say, grew out of the Conservative Party. There may be a heck of a lot of differences in what they believe, both in policy and in leadership, but they are still part of the conservative tradition.
The BNP might have certain beliefs in common with both UKIP and the Conservatives. But they are not part of either party, and they are not part of the conservative tradition