Mount Sinai was covered with smoke, because the Lord came down on it in fire. The smoke rose from the mountain like smoke from a furnace, and the whole mountain shook wildly.
And the Lord said to Moses, “I will come to you in a thick cloud and speak to you. The people will hear me speaking with you and will always trust you.” Then Moses told the Lord what the people had said.
The glory of the Lord came down on Mount Sinai, and the cloud covered it for six days. On the seventh day the Lord called to Moses from inside the cloud.
So be careful and do not refuse to listen when God speaks. Others refused to listen to him when he warned them on earth, and they did not escape. So it will be worse for us if we refuse to listen to God who warns us from heaven.