"He [Hillel] would say: ....Whoever does not study [Torah] is worthy of death, and whoever uses the crown [of Torah for personal gain] shall perish." (Pirkei Avot 1:13)

...the term "using the crown" refers to one who uses G‑d's ineffable name...

Rabbeinu Ovadia of Bartenura explains the phrase "And whoever uses the crown shall pass" refers to one who uses the crown of Torah the punishment for which is passing away from this world. He then writes as follows: "Additionally, I have heard that the term "using the crown" refers to one who uses G‑d's ineffable name, the punishment for which is passing on and being lost from the world, i.e. losing one's share in the World to Come." This refers to individuals who have learned from the books of the Kabbalists how to use the holy Divine Names in order to perform supernatural occurrences; such people have no share in the World to Come.

.... This teaches us the severity of using what is commonly referred to as "Practical Kabbalah," for even if there are individuals nowadays who may know how to use holy names for medical purposes and the like, one should abstain from consulting such sorcerers who may appear to be mystical and holy people. One who cherishes his soul should distance himself from such individuals who present themselves as Kabbalists when the extent of their integrity and righteousness is in serious question.

...this is not child's play that immediately upon reciting a specific holy name the sick person will be healed immediately.

Even if in truth they do possess some knowledge about holy names, it is reasonable to say that they are causing great damage by using these names, for this is not child's play that immediately upon reciting a specific holy name the sick person will be healed immediately. This is a deep and complex issue and whoever has only slight knowledge of this issue will end up causing much more damage that good. This is in addition to his great punishment and wickedness for his sin of profaning Divine Name in order to raise his own name.

If an individual requires a blessing for a certain salvation, he should seek out true Torah scholars who are the true authorities of Torah whom people respect and whose teachings and paths are logical and understandable, for G‑d will surely hear their prayers. Whoever cleaves to them and fears them is tantamount to cleaving to G‑d's holy presence; praiseworthy is such a person and his lot.

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[Translated by Rachmiel-Hayyim Drizin from //HalachaYomit.co.il]

Rabbi Ovadia Yosef of blessed memory is a former chief Rabbi of Israel and renowned authority in Jewish Law.