Is Kedusha a Biblical or Rabbinical obligation?
According to the Zohar[1], the daily recital of Kedusha is a Biblical command and is listed as one of the 613 Mitzvos.[2] One is thus Biblically required to hear Kedusha every single day. However, according to the majority of codifiers, the recital of Kedusha is a Rabbinical institution[3], and so is the implied opinion of Admur.[4] Nonetheless, even according to the latter opinion, one who recites Kedusha is fulfilling a Mitzvah of sanctifying Hashem’s name in public, which is even greater than a regular positive command.[5]

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[1] Raya Mihemna Emor 3 p. 93a, brought in Shaar Hakedusha 1/4, explained in length in Derech Mitzvosecha p. 34 “Mitzvas Amiras Kedusha Bechol Yom”; Shut Tzemach Tzedek O.C. 113 in name of Zohar and Shaar Hakedusha ibid and that so is implied from Brachos 21b which learns that Kedusha must be said with a Minyan from the verse of Venikdashti; This opinion is recorded to Halacha in: Beir Heiytiv 125/5; Ben Ish Chaiy Teruma 3; Aruch Hashulchan 125/4; Kaf Hachaim 125/4 in name of many Sifrei Kabala; Chofetz Chaim in Ahavas Chesed in end of Sefer

[2] This is learned from the verse “Venikdashti Besoch Bnei Yisrael”. [Zohar ibid; Tzemach Tzedek ibid; Ben Ish Chaiy ibid]

[3] Implied conclusion of Shut Tzemach Tzedek O.C. 113 based on majority of Rishonim who do not list Kedusha as one of the 613 Mitzvos. These include: Rambam; Ramban; Chinuch; Semag; Semak; Rosh; The above verse in the Gemara is a mere Asmachta; See Rishonim on Brachos 47b and Gittin 3b

[4] See Admur 90/17 “Although Davening with a Minyan is only Rabbinical”

[5] Admur ibid; This means that although we do not learn as does the Zohar that reciting Kedusha is a personal Biblical command upon each Jew, just like the Mitzvah of Tefillin and Sukkah, nevertheless, one who does recite it fulfills the Mitzvah of Kiddush Hashem. It is similar to the Mitzvah of Tzedaka, that although one is not obligated to give Tzedaka daily, if he does so he fulfills a Biblical command. Thus, the difference between the Zohar and codifiers is not that great.