Answer
Unity, Binity or Trinity, the One sitting on the throne according to Isaiah 6:1-3 is none other than Jesus Christ the Lord.
Explanation
Case # 1:-
No man has seen God the Father at any time in history. This is what the Scripture states emphatically:
“No one has seen God at any time; the only begotten Son, who is in
the bosom of the Father, that One declares Him” (John 1:18).
Yet we see in the Old Testament people seeing Yahweh God.
For example, Moses prayed to Yahweh to see Him in His “glory” unlike in human form when Abraham and Jacob saw Him:
“And he (Moses) said, I pray, let me see Your glory” (Exo 33:18).
Then what happened?
“And He (Yahweh) said, I will cause all My goodness to pass before
your face…….And as My glory is passing it will be that I will put you
in a cleft of the rock; and I will cover My hand over you during My
passing. And I will remove My hand, and you shall see My back” (Exo
33:19, 22-23).
There it is, Moses saw God in His real form!
But according to John 1:18, Moses did not see God the Father!!
Then whom did he see in Exodus 33:19-23?
Of course, he saw Yahweh who was the pre-Incarnate Jesus Christ the Lord.
How did the Son declare the Father according to John 1:18?
Let us see:
“And I (Yahweh) will call out (that is, declare) the name of
Jehovah (God the Father) before your face” (Exo 33:19).
“And Jehovah passed by before his face and called out (that is,
declared): Jehovah! Jehovah God (God the Father)! Merciful and
gracious, slow to anger, and great in goodness and truth……” (Exo
34:6).
We see, Yahweh declaring Yahweh, here.
But, John 1:18 says the only Son “declares” the Father!
Case # 2:-
No one has seen God the Father at any time:
“No one has seen God at any time” (1 John 4:12).
Yet, Moses, Aaron, Nadab, Abihu and 70 elders of Israel all saw Yahweh:
“And Moses and Aaron went up with Nadab and Abihu, and seventy from
the elders of Israel. And they saw the God of Israel. And under
His feet was as the work of a pavement of sapphire, and the same as
the essence of the heavens for clearness. And He did not stretch
out His hand to the nobles of the sons of Israel. And they saw
God, and they ate and drank” (Exo 24:9-11).
a) Who is the God of Israel? Yes, it is Yahweh alone. So, they saw Yahweh.
b) “pavement of sapphire under His feet” shows that this was a real sight.
c) “Yahweh did not kill them” shows that they really saw Yahweh.
d) “they ate and drank” shows clearly that this was not a vision or that they were not sedated.
So, the question is, if no man has seen God the Father any time, whom did these many people see, here?
Of course, they all saw Yahweh who was the pre-Incarnate Jesus Christ the Lord.
Case # 3:-
No man has seen God the Father at any time in history.
“not that anyone has seen the Father (God), except the One being
from God, He has seen the Father” (John 6:46).
Yet, we see that prophet Micaiah saw Yahweh “sitting on His throne”:
“And he (Micaiah) said, Therefore, hear the Word of Jehovah: I saw
Jehovah sitting on His throne, and all the host of Heaven were
standing by Him, on His right and on His left” (1 Kings 22:19).
But no one has seen God the Father at any time. So, whom did prophet Micaiah see here?
Of course, he saw Yahweh who was the pre-Incarnate Jesus Christ the Lord.
Son was sent by Father / Yahweh sends Yahweh
The New Testament Scripture says that God the Father sent the Son:
“that the Father has sent the Son as Savior of the world” (1 John
4:14).
The Old Testament Scripture says that Yahweh sent Yahweh:
“Come near to Me (Jehovah); hear this: I (Jehovah) have not spoken in
secret from the beginning. From its being, I (Jehovah) was there; and
now the Lord Jehovah, and His Spirit, has sent Me (Jehovah)” (Isaiah 48:16).
“I, Jehovah, will hasten it in its time. The Spirit of the Lord
Jehovah is on Me (Jehovah), because Jehovah has anointed Me
(Jehovah) to preach the gospel to the meek” (Isaiah 60:22-61:1).
“For so says Jehovah of Hosts: He has sent Me (Jehovah) after
glory, to the nations who plundered you; for he who touches you
touches the pupil of His eye. For, behold, I (Jehovah) will shake
My hand over them, and they shall be a prize for their servants. And
you shall know that Jehovah has sent Me (Jehovah)” (Zech 2:8-9).
Prophet Zechariah and Apostle John
Zechariah:
“Sing and rejoice, O daughter of Zion. For, lo, I (Jehovah) come; and
I (Jehovah) will dwell among you, says Jehovah. And many
nations (Gentiles) shall be joined to Jehovah in that day, and they shall be My people” (Zech 2:10-11).
John:
“He came to His own, and His own did not receive Him” (John 1:11).
[Hence the Gospel was declared to the Gentiles!]
Zechariah:
“And I (Jehovah) will dwell among you; and you shall know that
Jehovah of Hosts has sent Me (Jehovah) to you” (Zech 2:11).
John:
“Righteous Father, indeed the world did not know You, but I knew You;
and these have known that You sent Me” (John 17:25).
Prophet Isaiah and Apostle John
Isaiah:
“then I saw the Lord sitting on a throne, high and lifted up. And
His train filled the temple…….. And one cried to the other and said,
Holy, holy, holy is Jehovah of Hosts; all the earth is full of His
glory!” (Isaiah 6:1,3).
Just as Prophet Micaiah saw Yahweh “sitting on His throne” (1 Kings 22:19), Prophet Isaiah saw Yahweh “sitting on a throne” here.
Yet, no one has seen God the Father at any time.
John:
“Isaiah said these things when he saw His (Jesus’) glory,
and spoke about Him (Jesus)” (John 12:41).
Conclusion
Isaiah says he saw Yahweh sitting on the throne. John says Isaiah was talking about Jesus as the pre-Incarnate Yahweh as sitting on the throne.