Heresy means to oppose the beliefs held by the religion. Idolatry is to have deeply held beliefs that are foreign to your religion. Idolatry leads to heresy, because different belief systems are bound to conflict. And in Abrahamic faiths, idolatry is in itself heresy. But idolatry and heresy are different concepts, even though in Abrahamic religions it is hard to imagine one without the other.