How much "extra" can a person believe and be a Christian?

[Dr. Watson]

It seems to me (non-scholarly opinion here), that almost all heresies either a) have errant views on salvation or b) have errant views on the nature of God, most often, Christ. Or both.

I think that the creeds and confessions as formulated by the early church and maintained by orthodox Christians for centuries following are very good benchmarks for determining if one has heretical beliefs.