33 But it may be argued that the Apostle was not inspired by the Spirit of prophecy when he borrowed these prophetic words; that he was only interpreting at random the words of another man, and though, no doubt, everything the Apostle says of himself comes to him by revelation from Christ, yet his knowledge of the words of Isaiah is only derived from the book. I answer that in the beginning of that utterance in which it is said that the servants of the true God shall bless Him and swear by Him, we read this adoration by the prophet:— From everlasting we have not heard, nor have our eyes seen God, except You, and Your works which You will do for them that await Your mercy. Isaiah says that he has seen no God but Him.
For he did actually see the glory of God, the mystery of Whose taking flesh from the Virgin he foretold. And if you, in your heresy, do not know that it was God the Only-begotten Whom the prophet saw in that glory, listen to the Evangelist:— These things said Esaias, when he saw His glory, and spoke of Him. The Apostle, the Evangelist, the Prophet combine to silence your objections. Isaiah did see God; even though it is written, No one has seen God at any time, save the Only-begotten Son Who is in the bosom of the Father; He has declared Him, it was God Whom the prophet saw. He gazed upon the Divine glory, and men were filled with envy at such honour vouchsafed to his prophetic greatness. For this was the reason why the Jews passed sentence of death upon him.
Source: On the Trinity (New Advent)