Are Principles of Belief Subject to Rabbinical Interpretation?
R. Gil Student, in his essay reviewing Marc Shapiro's book on Principles of Faith(and discussed in a few places on his excellent blog), makes the following points, among others(and if I have misconstrued anything, I apologize):
1. In the Shulchan Aruch(code of Jewish law), those in certain categories cannot perform certain functions: the shechitah of an epicurian is not kosher, a Torah scroll written by a sectarian must be burned, one must not say amen to a blessing by a sectarian.
2. Because of these limits, a definition of these states in neccessary, ie we need to know what a sectarian is, what a epicurian is.
3. These definitions, in keeping with how Halachic issues are usually decided, are in the hands of the poskim-decisors.
4. Therefore, a posek can determine what is a sectarian and what is not, what beliefs constitute heresy and what is not heresy, and this definition can be different over time and place, depending on the posek and what he/they deem appropriate, of course, as long as it is within the bounds of tradition.
5. The conclusion is that not only are beliefs mandatory in Judaism, but that the mandatory content of that belief can be different, depending on the posek. Therefore, according to this line of reasoning, a ban such as the one that came out on R. Slifkin's books, declaring them to be heresy, is not only a reasonable Halachic outcome, but binding on those who choose to follow the rabbis who pronounced the ban.
Part II to follow(sorry, too much work)