Halakhah su Isaia 6:16

One should not say (to oneself): "I am now in an inner room - who can see me?", for the Holy One Blessed Be He, "His glory fills the whole earth" (Isaiah 6:3).

After that, U-va Le-Tziyon, also called Kedusha De-sidra, is recited. It contains the verses “Kadosh, kadosh, kadosh…” (Yeshayahu 6:3), “Barukh kevod Hashem mi-mkomo” (Yeḥezkel 3:12), and “Hashem yimlokh le-olam va’ed” (Shemot 15:18). The uniqueness of this Kedusha is that the verses are recited along with their Aramaic translations. The Sages ordained their recitation (even though they were already recited in Yotzer Or and in Kedusah of Ḥazarat Ha-shatz) so that every person praying would learn some verses of the Prophets every day. The verses are translated into Aramaic, so that the whole nation, which was fluent in Aramaic at that time the prayer was ordained, would understand their meaning.

Kedusha is recited in the third berakha of Ḥazarat Ha-shatz. The essence of the Kedusha is the congregation’s response with the verses: “Kadosh, kadosh, kadosh, Hashem Tzevakot, melo kol ha-aretz kevodo” (“Holy, holy, holy is the Lord of hosts; the whole world is filled with His glory”; Yeshayahu 6:3), “Barukh kevod Hashem mi-mkomo” (“Blessed be the Lord’s glory from His place”), and “Yimlokh Hashem Le-olam…” (“The Lord shall reign forever…; Tehilim 146:10). The custom today, based on Arizal, is for the congregation to recite the connecting passages as well. The ḥazan then repeats them aloud, and the congregation responds with the verses of the Kedusha (MB 125:2; Kaf Ha-ḥayim 2).

The root of this commandment does not need elucidation. The matter is known and revealed to all that this belief is the foundation of the religion, and that one who does not believe in it denies a fundamental principle and has no share and merit with Israel. And the content of the belief is that he fix in his soul that such is the truth and that a change of (about a change of) this in any way be impossible. And if he be asked about it, he will respond to any enquirer that his heart believes this and he will not assent to a change in it, even if they say they will kill him [as a result]. As all of this - when he takes the thing from the potential to actuality, by which I mean to say when he puts into the words of his mouth what his heart decides - strengthens and fixes the belief of the heart. And if one merits to rise in the levels of wisdom, 'his heart will understand and his eyes will see' with a sealed proof that this belief that he believed is true and clear - it is impossible that there be something besides this - then he will fulfill this positive commandment [in the best way].

He says “כתר²¹³The Keter, כתר, was an ancient beginning of the Kedushah, קדושה, or sanctification prayer. It is no longer said and Kedushah now begins with the word na'ariẓkha, נעריצך. It is part of the Sabbath and festival Musaf Service (see footnote 166).
The Kedushah said during the Musaf Amidah is different than the Sanctification recited in the Shaḥarit Amidah on the Sabbath (see footnote 17). The Musaf Sanctification dwells more sublimely on the majestic conception of the angels in heaven glorifying the Eternal King and it introduces Israel proclaiming in response to the angelic choir, the holiness, glory, unity, and sovereignity of God's name. The Shema, the congregation's confession of faith, is part of the Musaf Kedushah (see footnote 17, section (3)).
The Shema entered the Musaf Kedushah as the result of a sixth century persecution. The Byzantine Empire forbid Jews to recite the Shema in public worship. On Sabbaths and festivals government spies would enter the synagogues to make sure the Shema was not said in its regular place during the Shaḥarit Service. As a means of getting the Shema into public prayer, the reader would insert it into the Musaf Kedushah, a place where the spies would not expect it and thus not be aware of it. The Shema has remained a part of this Sanctification ever since.
The Kedushah is part of the third blessing of the Amidah and in the Musaf Service for the Sabbath it is made up of the following parts along with connecting sentences: Isaiah 6:3, Ezekiel 3:12, the Shema, Deuteronomy 6:5, and Psalms 146:10. On festivals there is an additional verse from Zechariah 14:9 following the Shema.
Hertz, op. cit., pp. 528-31; 816-19.”, “the crown”, just as in the Additional Service.²¹⁴Musaf, מוסף; see footnote 166.