Every week of the year has its own Torah portion (a reading from the first five books of the Bible) and its own haftarah (an accompanying reading from the books of the prophets). This week the Torah portion is Yitro (Exodus 18:1-20:23), and the haftarah is Isaiah 6:1-7:6 & 9:5-6.
You cannot see My face, because humankind may not see Me and live. (Exodus/Shemot 33:20)
Although God cannot be seen directly, people in the Bible do experience visions of God. The Israelites see a manifestation of God in this week’s Torah portion, and Isaiah sees a manifestation of God in this week’s haftarah.

In the vision shared by everyone at Mount Sinai, God appears only as fire.
And Moses brought out the people from the camp to meet God, and they stationed themselves at the bottom of the mountain. And all of Mount Sinai smoked, because God went down upon it in fire, and its smoke went up like the smoke of the furnace, and all the mountain shuddered very much. (Exodus 19:17-18)
A shuddering, smoking mountain could be a volcano—except that in this vision, God’s fire comes down from the sky, not up from a crater. God also manifests in the book of Exodus as the fire Moses sees in the burning bush on Sinai, as the pillar of cloud and fire that leads the Israelites to Mount Sinai, and as fire and cloud on the mountain when Moses ascends to receive each pair of stone tablets.
No angels or other semi-divine creatures appear in the revelation at Mount Sinai; only fire, smoke, and various sounds.
In Isaiah’s moment of revelation, God does not appear as fire.
In the year King Uzziyahu died, I saw my Master sitting elevated on a lofty throne, and His skirts filling the heykhal. (Isaiah 6:1)
heykhal (הֵיכָל) = palace, temple; main room of the temple in Jerusalem; heavenly palace.
Isaiah beholds God wearing a robe and sitting on a throne, like a king—except that the skirts of the robe mysteriously flow out to fill the room. As the vision continues God speaks, but does not move.
However, angelic attendants surrounding God move, speak, and burn with fire.
Serafim are stationed above Him, each with six wings; with one pair he covers his face, and with a pair he covers his raglayim, and with a pair he flies. And one calls to another, and he says: Holy, holy holy! God of Tzevaot, Who fills all the earth with His glory! And the supports of the threshold shiver from the sound of the calling, and the house fills with smoke. (Isaiah 6:1-4)
serafim (שְׂרָפִים) = burners, burning creatures. (From the verb saraf, שָׂרַף = burn. Used in Numbers and Deuteronomy for “burning serpents”—probably poisonous snakes.)
tzevaot (צְבָאוֹת) = armies (on earth); the stars (in the heavens).
raglayim (רַגְלָיִם) = (pair of) feet, legs; a euphemism for the penis. Singular: regel (רֶגֶל) = foot, leg; walking pace; time set for a pilgrimage-festival.
The serafim must both cover and uncover their faces and their raglayim; if these body parts were permanently covered, they would not need wings for that purpose. When and why do they conceal these parts of their anatomy?
In Leviticus Rabbah, a collection of commentary from 400-600 C.E., Rabbi Jacob ben Zadbi says the serafim cover their faces to avoid looking at God’s presence, and cover their feet so God would not have to look at such unsightly appendages. (The writer assumed that the feet of the serafim were like the feet of Ezekiel’s angelic keruvim, which resembled calves’ feet.)
Twelfth-century C.E. rabbi Moses Maimonides wrote in The Guide for the Perplexed that the description of serafim covering their body parts is symbolic. The faces of the serafim are covered to indicate that “the cause of their existence is hidden and concealed”, while their feet are covered to indicate that their actions in the universe are also hidden. The wings for flying, Maimonides adds, merely represent the speed with which the serafim move when they act.
I propose a simpler explanation. Maybe the serafim cover their faces whenever they turn toward Isaiah, so he is not exposed to the blinding light radiating from these burning creatures. If seeing God’s face means death, seeing the faces of the serafim might be almost as bad.

As for covering their raglayim, I doubt the serafim are concealing their feet. After all, humans must have bare feet when they are in God’s presence; Moses must remove his sandals in front of the burning bush, and the priests must go barefoot inside the sanctuary. Since Isaiah’s vision is set inside a heykhal, the serafim in God’s presence probably expose their bare feet.
Although the word raglayim most often refers to feet or legs, sometimes it implies the pubic area between the legs, and there are three places in the Bible where raglayim is definitely a euphemism for the male genital organs. In Judges 3:24 and 1 Samuel 24:4 the word raglayim is combined with a verb to indicate a man urinating. And in the part of chapter 7 of Isaiah that is left out of this week’s haftarah, the prophet says that God will use the king of Assyria as a razor to shave off the head of hair and the hair of the raglayim (JPS: public hair) and also snatch away the beard. (Isaiah 7:20)
A man’s hair, especially his beard and pubic hair, stood for virility in ancient Israelite culture. Isaiah employs a shaving metaphor to prophesy that God will use Assyria to symbolically castrate Israel’s other enemies.
Why would the serafim in Isaiah’s vision use their extra wings to cover their genitals?
The penis is a symbol of rule, dominance, and control throughout the Bible, from the oath Abraham’s servant swears on his master’s yareich (which can also mean genitals; see my post Chayyei Sarah: A Peculiar Oath) to the Persian king who approves Esther’s interruption by lifting his sharvit (scepter). But God is the ultimate ruler. It would be subversive for a male to uncover his genitals in God’s presence.
That is why this week’s Torah portion specifies that all altars for God must be built without stairs or steps.
You must not ascend on stairs to My altar; that way you would expose your nakedness upon it. (Exodus 20:23)
The Torah also requires that priests must wear linen undergarments, so their genitals will be concealed in all areas of God’s sanctuary.
So each seraf uses one pair of wings to conceal his fiery face from Isaiah, for his own protection; and one pair of wings to conceal his genitals, so Isaiah will know that God rules, not the serafim.
Nevertheless, these angels are endowed with the potential to generate independent decisions and actions. One example occurs after Isaiah expresses his anxiety about having a vision of God.

And I said: Woe to me! I am as good as dead, because I am a man of impure lips, and I am living in the midst of a people of impure lips, yet my eyes behold the King, God of Tzevaot! Then one of the serafim flew toward me, and in his hand was a live coal he had taken from the altar with a pair of tongs. And he touched it to my mouth and he said: Hey! Now that this has touched your lips, your bad deeds have gone away, and your offense is atoned for. Then I heard the voice of my Master saying: Whom shall I send, and who will go for us? And I said: Here I am, send me! (Isaiah 6:5-8)
We have come a long way from the vision at Mount Sinai of God as undifferentiated fire, unaccompanied by any furniture or subsidiary creatures.
Isaiah sees God in terms of a throne and skirts, not in terms of fire. The fire exists in God’s serafim, “burning ones”, who occupy a station somewhere between humankind and God. They praise God (Holy, holy holy!) and they are privy to some of God’s plans (and who will go for us?) They protect Isaiah from the blinding brightness of their faces, and they cover their genitals to indicate that although they have some power, God is the ultimate ruler. And one seraf, hearing Isaiah’s anxiety about his unworthiness, takes action to remove his guilt. In his relief, Isaiah volunteers to be God’s prophet. Thus the seraf both furthers God’s plan and helps Isaiah rise to his calling.
The image of God as a king with a throne and a long robe has continued to be popular, from some of the writings after the fall of the first temple to some of the explanations given to children today. For me, God as fire is a better metaphor. An individual human cannot become a god.
But maybe we can aspire to be brighter, more aware of God’s presence, and more able to listen to people and address their concerns.
May all of us humans learn to act as thoughtfully as the serafim in Isaiah’s vision.