“On the morning of the third day there was thunder and lightning, as well as a thick cloud on the mountain.  The blast of a trumpet was so loud that all the people who were in the camp trembled.  Moses brought the people out of the camp to meet God.  They took their stand at the foot of the mountain.  Now Mount Sinai was wrapped in smoke, because Yahweh had descended upon it in fire.  The smoke went up like the smoke of a kiln, while the whole mountain shook violently.  When the blast of the trumpet grew louder and louder, Moses would speak.  Then God would answer him in thunder.  When Yahweh descended upon Mount Sinai, to the top of the mountain, Yahweh summoned Moses to the top of the mountain, and Moses went up.  Then Yahweh said to Moses, ‘Go down and warn the people not to break through to Yahweh to look.  Otherwise many of them will perish.  Even the priests who approach Yahweh must consecrate themselves or Yahweh will break out against them.’  And Moses said to Yahweh, ‘The people are not permitted to come up to Mount Sinai.  You yourself warned us, saying, set limits around the mountain, and keep it holy.’  Yahweh said to him, ‘Go down, and come up bringing Aaron with you.  But do not let either the priests or the people break through to come up to Yahweh.  Otherwise, he will break out against them.’  So Moses went down to the people and told them.”

This is an example of a theophany, an appearance of God that is very rare.  Only the holy people get to speak directly with Yahweh.  Yahweh came in a thick cloud with thunder and lightning on the mountain.  There was a loud trumpet blast.  This is a big deal as God appears in thunder and lightning.  Yahweh comes from above and rests on the mountain.  This is the classic concept that up is good and down is bad.  The good up God is powerful and speaks down to the people.   With the blasts of the trumpet louder, Moses would speak and Yahweh would respond using thunder.  Yahweh summoned Moses to the top of the mountain, and Moses went up.  Then Yahweh told him to get Aaron and bring him up to the mountain top.  However, Moses was not to let any other people up, even the priests.