Jesus warned us to stay away from the leaven of the Pharisees. I don't think he defined that as bad teachings, but rather as a spiritual attitude on their part. In fact, he affirmed their teachings. So if we are going to warn brothers, let's warn them about their spiritual attitudes, not their beliefs. Dear Doug, Thanks for your message.
Heresy is a powerful word and one that no one likes attached their person, present or past.
We would rather not describe anyone as a heretic, and we certainly do not want to be described as one ourselves. Heresy is generally defined as religious teaching which is unbiblical and outside the core teachings of the historic, orthodox Christian faith. This teaching may be accepted and held by one in ignorance, or in apostasy - which would be the abandonment of the Christian faith. On one hand the end result is the same. Whether you ignorantly accept false teaching, or whether you deliberately accept it, fashion it, fabricate it, etc.