After three months of travelling in the desert, God's people arrived at Mount Sinai, where they would stay an entire year as God gave Moses his laws of relationship with him, with each other, with strangers and even with animals; instructions on how to build a house of worship where his people could come to him and the actual construction of the tabernacle and all it needed to function.
Soon after their arrival to Mt. Sinai, God wanted to meet with his people but there were things they needed to do first: be consecrated by Moses, wash their clothes, abstain from sexual relations and be ready. There was a line around the mountain they could not cross. If even an animal crossed the line, it was to be killed in a way that did not involvecrossing the line or touching the animal or person to be killed.
On the big day, there was thunder and lightning. A thick cloud covered the mountain and a loud trumpet blast sounded across the entire camp. The people shook in fear as they began approaching the mountain and what they saw made them tremble even more. The mountain itself was trembling. The trumpet increased in volume and smoke billowed as God descended to the top of the mountain in fire.
Moses went up the mountain and there God told him to warn the people that he, God, would break out against them if they crossed the line. God gave the warning twice. Meeting God was a perilous business!
Hebrews 12:18-29 uses the story above to givethe New Testament view of approaching God. It's worth reading.