Oh there are a lot of Christian groups that don’t believe the same as Mainline Protestant. Mainline Protestant is but a group. There are other groups and some that stand alone as well.
“evangelical,
fundamentalist,
and charismatic”
are three categories that AREN’T considered “mainline Protestant Christian”.
Mormons I think stand uniquely in their own category.
Then there are the Roman Catholics, then there are the Eastern Orthodox who are also their own categories