Great, then "Christian" only applies to those who believe in the trinity.
Every Christian believes in the "Trinity".....
However, not every Christian believes in the Nicene Creed, nor have they from the beginning, especially when the Nicene Creed (aka Trinity) was officialized by the Catholic Church, not "Christians" in general.
A good half of Christiandom did not believe in the Nicene Creed when it was established.
Yes, some did fall in line after, but not all.
Even further, would it surprise you that even Mormons are able to believe in the "original" Nicene Creed?
It actually doesn't really say anything contrary to LDS theology, depending on how it's interpreted. It's the later interpretations and re-writing that makes it wrong. In it's more generic original renditioning, it was able to allow various views of the matter. For example, the "one substance" statement. To an LDS, of course Christ and the Father are "one substance", because they are literally Father and son genetically speaking, because the Father contributed his DNA. Of course, that term/interpretation was over-ridden by a false one, which became the dominant interpretation, that substance means they are the same being.