“Holy, holy, holy”
Isaiah sees the throne, and the seraphim call to one another: qadosh, qadosh, qadosh. Hebrew has no word for “holiest.” To say a thing utterly, Hebrew says it again — and to say it beyond that, it says it three times. Isaiah 6 is the place readers usually meet that habit of the language for the first time.
John hears the same threefold cry in Revelation, in Greek, around the same throne.
One more thing worth knowing about qadosh: its root idea is not “morally spotless” but “set apart for one use only.” That is why the same three letters can produce the holy God, the holy place, and the holy vessels — things that belong to one purpose and no other.
“And they were calling out to one another: “Holy, holy, holy is the LORD of Hosts; all the earth is full of His glory.””
Isaiah 6:3→
“And each of the four living creatures had six wings and was covered with eyes all around and within. Day and night they never stop saying: “Holy, Holy, Holy, is the Lord God Almighty, who was and is and is to come!””
Revelation 4:8→