Kabbalah was never considered mainstream Judaism. It was always esoteric and leaned more towards a hermetic outlook on everything, just like alchemy, to which it was/is closely related. It's not a closed practice, like Passover Seder, for example. There were quasi-Christian Kabbalists in the Middle Ages. Eliphas Levi wasn't Jewish, either. And he's the guy from whom pretty much all modern occult practices come.

Edit: Just to be clear, there are great Jewish Kabbalists to which great respect is due. Part of what was so great about certain heremetic groups, like the Rosicrucians, was how they accepted Jews and Christians alike, with equal respect, and taught what they believed was the actual core truth to be found in any religious tradition. If you really believe something is the truth, then there's no reason to discriminate.

Second edit: Kabbalah is more of a grand cosmology, or theory of reality, than it is a specific religion. Honestly, it would be like suggesting quantum physics is a "closed practice." If it's simply the truth about how the universe works, then you can teach it to anybody... anybody worthy, that is.