Here is a post from Justin Holcomb on dealing with heresy and what it is not. The whole thing is well worth the read, but here are some snippets.
If a believer authentically holds to the Nicene Creed, we should not call them a heretic, no matter how strongly we believe they are gravely in error on the details or on other doctrines. A good shorthand for heresy, then, is to ask, “Can they say the Nicene Creed and mean it without their fingers crossed?” If the answer is yes, they may still be wrong, and they may be heterodox, but we cannot call them heretics, because they fit within the bounds of historic Christianity.
For the grammatically anal, it should “if a believer holds….we should not cal HIM (not them) a heretic.” But that’s beside the point. I love his winsome and humble attitude, which I think is what Paul is driving at in Phil 4:8 in “whatever is lovely.” This attitude is an absolute necessity, though rare, in theological dialog. This guy went to Reformed Theological Seminary (where I went) and is doing a fine job at being “winsomely reformed,” as they taught us down in those parts. You don’t have to be a jerk to question, challenge, discern, or dialog with those of differing theologies-even bad ones.
Such an attitude of humble, charitable engagement stands in stark contrast to the spirit of the blogosphere today. Rather than being fundamentalists who turn disagreement into division, we should contend for the truth with humility and grace. That’s how Jesus treated us.