It is the idea that one is not supposed to make up a new religion.
That means in practical terms that even though what does it means to keep the Torah can be hard to decide on a daily basis still we know what it means not to keep the Torah.
That means to say we know more or less that a Jews is supposed to learn the Oral and written law--the Five books of Moses and the Babylonian Talmud from cover to cover- and to keep what the Torah says. Period.
But in this process sometimes people have experience with other individuals which might not be optimum. This still does not give one permission to go and make up some new religion.
Learning Torah has been considered the prime directive of the Torah for a couple of thousand years. This is not the subject of any debate. That means to say at minimum everyone should sit down a learn Gemara, Rashi, and Tosphot at least an hour every day. And when it comes to Jewish law, the requirement to keep it is not a debate-- although the particulars are.
Also Monotheism is the philosophy of the Torah. This also has never been the subject of ant debate. Torah is not a document of pantheism in any sense and no one ever considered it as such--not Maimonides nor Isaac Luria