I believe in the concept of heresy. To be a Christian, one must believe in a core set of ideas (what some refer to as “primary doctrines”). If you deny or sufficiently distort those doctrines, you are not a Christian and will not be saved.
While there are disagreements about which doctrines qualify as primary, most would agree that the doctrine of God, the doctrine of Christ, the doctrine of salvation, and the physical resurrection of Jesus are on that list. Most people would also agree that there is some latitude for disagreements on these issues, but nobody agrees on just how much latitude can be tolerated before one moves from the realm of orthodox to the realm of heresy. For example, many consider Nestorianism to be a Christological heresy, whereas others, such as myself, have argued that it should only be considered a Christology error. In other words, I think the doctrine of Christ is flexible enough that a Nestorian can still be considered a Christian and saved, despite his theological error.