For the reasons Vermin05 describes: if you don't believe in the Trinity, you are not, by definition, a Christian. That's been settled for something like 1700 years.

They asked me what they should believe

Mormons don’t, but a lot of us believe believing in the Holy Trinity is what makes you Christian.

door believes in the existence of the holy trinity either .

Maybe there’s a correlation between religions that bother people in their houses and non-trinitarianism.