Hillel emphasizes that not treating others in a manner that we do not wish to be treated is truly Judaism's essence, and not just one of several important principles of behavior. Thus, although belief in monotheism is at the heart of Judaism, if someone believes in God but does not practice Hillel's guideline in his dealings with others, he cannot be regarded as a religious Jew. Indeed, we may question if the God in whom such an individual believes is the God in whom Judaism believes.
— Joseph Telushkin, A Code of Jewish Ethics