Sanctifying God's name, like the angels.
Reprinted from The Jewish Religion: A Companion, published by Oxford University Press.
Kedushah is the sanctification of God's name during the Reader's repetition of the Amidah. During the Reader's repetition, when he reaches the third paragraph, the theme of which is God's holiness, he declares: 'We will sanctify Thy name in the world even as they sanctify it in the highest heavens, as it is written by the hand of Thy prophet: "And they called one unto the other and said, Holy, holy, holy is the Lord of hosts; the whole earth is full of His glory" (Isaiah 63).'
From 'Holy, holy, holy' onwards is chanted by the congregation. The Reader continues: 'Those over against them say, Blessed,' to which the congregation responds: 'Blessed be the glory of the Lord from His place (Ezekiel 3:12).' The Reader continues: 'And in Thy holy words it is written, saying,' to which the congregation responds: 'The Lord shall reign forever, thy God, O Zion, unto all generations. Hallelujah (Psalms 146:10).'
The Kedushah is thus a re-enactment by the congregation on earth of the angelic praising of God on high. In one Talmudic passage it is stated that the angels do not begin their song until Israel has recited the Shema on earth and that, moreover, the divine name occurs in the Shema after only two words ('Hear' and 'Israel') whereas the angelic hosts are only permitted to give utterance to the divine name after three words ('Holy, holy, holy').
Shema in Kedushah
From the sixth century CE the Shema is added to the Kedushah in the Musaf (Additional) Prayer on the Sabbaths and festivals. This is reportedly because, the Byzantine authorities, whether Christians or Persian dualists, would not permit Jews to declare publicly the unity of God in the Shema.
Consequently, the Shema was not recited in the usual place in the early morning service but only in the later Additional Prayer in order to avoid the watchful eyes of the governmental authorities. There is probably some historical truth behind this report.