Isa 6:1 In the year that King Uzziah died, I saw the Lord seated on a throne, high and exalted, and the train of his robe filled the temple. 2 Above him were seraphs, each with six wings: With two wings they covered their faces, with two they covered their feet, and with two they were flying. 3 And they were calling to one another:
“Holy, holy, holy is the LORD Almighty;
the whole earth is full of his glory.”

I saw the Lord – Whom did Isaiah see?  It is usual, because of the “Holy, holy, holy is the Lord Almighty” which follows, to regard this as a reference to the divine Trinity.  However, some doubt is introduced by John 12:41, where Jesus, after quoting Isa 6:9f, alludes to the present verses by saying:

“Isaiah said these things because he saw Christ’s glory, and spoke about him.”

Reading back from the standpoint of a distinctively Christian theology, Calvin insists that it would be wrong to limit the reference in this passage to Christ alone, but rather to God without differentiation.  Herman Witsius argued that, because the NT not only links this passage with Christ (Jn 12:41, mentioned above), but also with the Holy Spirit (Acts 28:25-27), it would be unreasonable to suppose that the Father himself were to be excluded.  He concludes that Isaiah saw the whole Trinity in his vision.  See this discussion by Derek Rishwamy.

But is it possible to 'see God'?
What about Jn 1:18, where it is written that ‘no one has seen God’?

Ian Paul responds by saying that the apparent contradiction is actually quite widespread in Scripture:

‘Moses was said to have talked with God face to face, and he and the elders of Israel sat with God and ‘ate and drank’. And Jesus (in the Beatitudes) promises that the pure in heart will ‘see’ God. But what do we mean by ‘see’? (This is a very pertinent question in relation to the Book of Revelation.) Even in ordinary language, we use ‘see’ in all sorts of different ways—’I see what you mean’; ‘I suddenly saw the answer’; ‘I can see the sea’. Much of the time, the Bible is  using metaphorical and visionary language. In Isaiah 6 (as in Revelation) it is hard to take this literally—how could God be ‘high and lifted up’ and have his ‘train fill the temple’ unless the temple’s roof was taken off? The important thing is that, in the year that King Uzziah (who’s name means ‘God is my strength’) died, that is, at a time of change and uncertainty, God is still the one who is on his throne, and for Isaiah the truth of that was found in his temple presence. Scripture is clear that God is beyond human comprehensive—we see only in part—and that is why God is never actually described in Revelation, but simply referred to as “the one seated on the throne.”‘

Without holiness . . .

His patience would be an indulgence to sin,
His mercy a fondness,
His wrath a madness,
His power a tyranny,
His wisdom an unworthy subtlety.

(Stephen Charnock)

Isa 6:4 At the sound of their voices the doorposts and thresholds shook and the temple was filled with smoke.
Isa 6:5 “Woe to me!” I cried. “I am ruined! For I am a man of unclean lips, and I live among a people of unclean lips, and my eyes have seen the King, the LORD Almighty.”
Isa 6:6 Then one of the seraphs flew to me with a live coal in his hand, which he had taken with tongs from the altar. 7 With it he touched my mouth and said, “See, this has touched your lips; your guilt is taken away and your sin atoned for.”
Isa 6:8 Then I heard the voice of the Lord saying, “Whom shall I send? And who will go for us?”
And I said, “Here am I. Send me!”
Isa 6:9 He said, “Go and tell this people:
“‘Be ever hearing, but never understanding;
be ever seeing, but never perceiving.’
Isa 6:10 Make the heart of this people calloused;
make their ears dull
and close their eyes.
Otherwise they might see with their eyes,
hear with their ears,
understand with their hearts,
and turn and be healed.”
Isa 6:11 Then I said, “For how long, O Lord?”
And he answered:
“Until the cities lie ruined
and without inhabitant,
until the houses are left deserted
and the fields ruined and ravaged,
Isa 6:12 until the LORD has sent everyone far away
and the land is utterly forsaken.
Isa 6:13 And though a tenth remains in the land,
it will again be laid waste.
But as the terebinth and oak
leave stumps when they are cut down,
so the holy seed will be the stump in the land.”