‘And Mount Sinai was totally covered in smoke because Yahweh descended on it in fire, and its smoke ascended as the smoke of a furnace and the whole mountain quaked greatly.'
Fire and smoke were both symbols of God in Genesis 15:17 where He appeared in a smoking furnace and a flaming torch. This manifestation at Sinai reproduced the same on a grander scale. It would remind the people of that covenant with Abram, confirmed by smoke and fire, and the great deliverance from Egypt He then promised (Genesis 15:13-14). Fire was the most awe-inspiring thing known to ancient man, and fire swirling with smoke the most destructive. Man had experienced its effects often, he had seen it consume great areas of land, he knew himself how to use it for destructive purposes. And he had learned to fear it. And God was the untouchable and unapproachable fire.
God appearing in the likeness of fire is common in both Old and New Testaments (see Genesis 15:17; Exodus 13:21; Exodus 19:16; Exodus 19:18; Exodus 20:18; Exodus 24:17; Exodus 40:38; Deuteronomy 4:11; Ezekiel 1:27; Ezekiel 8:2: Act 2:3; 1 Timothy 6:16; Revelation 21:23; Revelation 22:5). To the ancient such a manifestation showed God to be a combination of the inexplicable and the beneficial, of the dangerous and yet vital. Yahweh had no form and yet like fire could be seen even in the darkness. He could even bring light to the darkness. Like fire He benefited man and yet could consume him. He was glorious and awe-inspiring and then in a moment He could be gone. In manifestation fire brought home something of the significance of the divine. But above all it revealed glory.
“Totally covered in smoke --- its smoke ascended as the smoke of a furnace.” Isaiah 4:5 demonstrates that smoke and cloud can be seen as parallels in this context, for he spoke of ‘a cloud and smoke by day and the shining of a flaming fire by night' with the pillars of cloud and fire, and probably this scene, in mind (compare also Isaiah 6:4). Thus the smoke represented Yahweh appearing in a cloud.
But the ascending smoke would also remind them of that dreadful and awesome day when God reigned down fire from heaven on Sodom and Gomorrah. There too ‘the smoke of the land went up like the smoke of a furnace' (Genesis 19:28). He was a God of judgment as well as a God of glory. So we have both fire and smoke, light (Exodus 13:21) and judgment, the Guide (through cloud and fire) and the Judge.
“The whole mountain quaked greatly.” This may have been caused by the impression given by the excessive rumbling of the heavy thunder and the continual flickering of the lightning, heightened by the divine presence, or there may have been a minor earthquake that shook the mountain to its core. Either way it added to the awesomeness of the experience.