I think I may soon have to expand on the actual definition of a much-misunderstood and abused term: heresy. In historical theology, "heresy" has a very specific referent that has nothing to do with whether or not a belief is "true" or "Biblical." Instead, the term refers to teaching that is out of line with the creeds written by the ecumenical councils. You don't have to like that, but that is what the term means. Most Protestants today struggle with understanding this (perhaps due to an anti-Catholic hangover?) and have altered the meaning without realizing it.
Thus, what constitutes “heresy” and orthodoxy evolves over time. For example, the Cappadocian Fathers, who are responsible for the chief defense of the Trinity in the early church formulation (which is the base for orthodoxy now), appear to be universalists. A few hundred years later, universalism becomes a heresy, which puts us in the awkward situation of calling the Cappadocians both the paragons of orthodoxy as well as heretics at the same time. To take that further, the 4th Lateran council makes a literal, everlasting fiery hell a standard for orthodoxy. So if you believe there is a hell, but you believe it isn’t necessarily a fiery place, congratulations, you are a heretic.*
The first significant creed in the early church is the Apostles Creed. Today, John Piper posted a refutation of the line in the Creed that says Jesus "descended into hell" (greek: hades). Piper writes:
“… it seems best to me to omit from the Apostles Creed the clause, “he descended into hell,” rather than giving it other meanings that are more defensible…” - John Piper
Oops. That does technically make you a heretic. Hello, Rob Bell!
*Of course, I think the idea of hell is not only ridiculous, but tragically harmful. That makes me a heretic on this point. I try to be honest about that.