The "standard" beliefs of Christianity are expressed in the Nicene creed[1][2]. Even recent evangelical churches believe the things listed there, although they probably don't recite the creed itself.

Most mainstream Christians don't consider Mormons as Christians because they have added to the Bible, which is considered a heresy.

[1] https://en.wikipedia.org/wiki/Nicene_Creed#Comparison_betwee... (note that "catholic" in this means "diverse" and doesn't specifically refer to the Catholic church)

[2] the differences between the two creeds listed there are basically what split the Orthodox church from the Western church and indirectly led to the fall of Constantinople. So some people consider them fairly important, but others don't.