Sacrilege: Please don’t mis-characterize. The RAMBAM (among others), not the entire Jewish religion, holds that the thirteen ikkarim are core essential beliefs. (Though even according to him, these may not be * beliefs* per se, as much as actual knowledge and conviction).
The Maharal, based on some of the Geonim, and R. S.R. Hirsch both rejected the idea that any beliefs are required to be a Torah-true Jew. For them, correct actions were paramount. Indeed, there are numerous gemaros that indicate the correctness of that view. (e.g. Berachos, Third perek, I think, where the chachamim abolished saying the aseres hadibros, and see Rashi ad. loc.).