Isaiah 6:3
Isaiah 6:33And they were calling out to one another: “Holy, holy, holy is the LORD of Hosts; all the earth is full of His glory.”
"Holy, holy, holy is Jehovah of armies. The whole earth is full of his glory!" The Watchtower reads the threefold "holy" as a Hebrew superlative — emphasis through repetition — and rejects any Trinitarian inference. The verse is treated as a strong affirmation of Jehovah's transcendent purity, with no implications for the identity of the figure on the throne beyond confirming it is Jehovah alone.
is the throne-room exclamation that redirects: "Isaiah said these things because he saw his glory and spoke about him." The "him" — by 's grammatical chain — is Jesus. The seraphim's "holy, holy, holy" is therefore ascribed, in apostolic interpretation, to Jesus. Pair ("I saw Jehovah sitting on a lofty and elevated throne") with ("Isaiah saw his glory" — meaning Jesus's). One person Isaiah saw; one person John identifies as Jesus; one glory the seraphim cried over. The doorstep form: "Whose glory did Isaiah see — Jehovah's or Jesus's? gives the apostolic answer."