My basic idea is to concentrate of what the Torah and the Talmud Bavli say. But given the wide range of interpretations that are possible, I look for rigorous thinkers like Maimonides and Saadia Gaon and the Chovot Levavot [Duties of the Heart by Bacheye ben Pekuda] to define what is in the category of Torah and what is not.
I should mention that heresy is in fact an important theme in the Talmud. While we do not find the thirteen principles of Maimonides spelled out in that exact order and formulation, we do find all of them stated openly and it is stated in the Talmud that one that does not believe in them does comes under a general classification of "apikorus" or "min." [Heretic.]
Many people have attempted for two centuries to bring pantheism into Torah. But according to Maimonides the Torah is not pantheistic. It is Monotheistic. There is a difference between God creating the world and a god being the world. This is entailed by the law of the excluded middle.
Others try to bring in moralrelativism. But moral relativism is surely false as can be demonstrated from two trivial axioms, namely, the law of excluded middle and the correspondence theory of truth. For if moral judgments represent claims, then we know from the law of excluded middle that they must be either true or false. That is just basic logic. And if they are true, then we know from the correspondence theory that that means they correspond to reality. And, finally, if they correspond to reality but they don't correspond to the nature of the object then they must correspond to the nature of the subject. But this last alternative is not true. we don't think if all would think Nazism is good that that would make it good.
More so--the type of moral subjectivity is in violation of the naturalistic fallacy. You cant derive an "ought" from an "is." That fact that one or even many say something is moral is an empirical fact. You cant derive an a priori "ought" from that fact.