"And Moses went up to God, and the LORD called to him from the mountain, saying, 'Thus you shall say to the house of Jacob, and tell the children of Israel: "You have seen what I did to the Egyptians, and how I bore you on eagles' wings and brought you to Myself. Now therefore, if you will indeed obey My voice and keep My covenant, then you shall be a special treasure to Me above all people; for all the earth is Mine. And you shall be to Me a kingdom of priests and a holy nation." These are the words which you shall speak to the children of Israel'" (Exo. 19:3-6).
God continued to instruct Moses the mediator regarding what to say to the people. Jehovah intended to enter into a special covenant with the nation of Israel. He had demonstrated His omnipotence to them (cf. Deut. 32:9-12) and desired to bless them as a special people. However, He would require them to be faithful (a condition man has always struggled with). There are some interesting parallels here between Israel and the New Testament church. Most significant is the fact that God still has a special priesthood of people today (i.e., Christians; cf. I Pet. 2:9,10) and He blesses them in a unique way (cf. Eph. 1:3).
"Then all the people answered together and said, 'All that the LORD has spoken we will do.' So Moses brought back the words of the people to the LORD. And the LORD said to Moses, 'Behold, I come to you in the thick cloud, that the people may hear when I speak with you, and believe you forever.' So Moses told the words of the people to the LORD. Then the LORD said to Moses, 'Go to the people and consecrate them today and tomorrow, and let them wash their clothes. And let them be ready for the third day. For on the third day the LORD will come down upon Mount Sinai in the sight of all the people'" (Exo. 19:8-11).
The people say "yes" to God's leadership. Though it should not have been needed, more divine proof of God's approval of Moses will be shown. The people were to be consecrated; that is, set apart for a special purpose. They were about to meet God! They needed to prepare themselves for this special relationship they are about to enter into. Sanctification is never without conditions. In this case, the people were instructed to do three things: (1) wash their clothes, (2) set boundaries around the mountain so no one would touch it, & (3) abstain from sex (cf. I Cor. 7:5). These rules applied to everyone for three days. And then God would make His glory known!
"Then it came to pass on the third day, in the morning, that there were thunderings and lightnings, and a thick cloud on the mountain; and the sound of the trumpet was very loud, so that all the people who were in the camp trembled. And Moses brought the people out of the camp to meet with God, and they stood at the foot of the mountain. Now Mount Sinai was completely in smoke, because the LORD descended upon it in fire. Its smoke ascended like the smoke of a furnace, and the whole mountain quaked greatly. And when the blast of the trumpet sounded long and became louder and louder, Moses spoke, and God answered him by voice. Then the LORD came down upon Mount Sinai, on the top of the mountain. And the LORD called Moses to the top of the mountain, and Moses went up. And the LORD said to Moses, 'Go down and warn the people, lest they break through to gaze at the LORD, and many of them perish. Also let the priests who come near the LORD consecrate themselves, lest the LORD break out against them'" (Exo. 19:16-22).
The people were understandably terrified. Can you imagine this scene: thunder, lightning, a thick cloud, an increasingly loud trumpet, smoke, & fire (cf. Heb. 12:18-21)? What an awesome sight it would have been! God won't begin speaking to the people until Moses goes down one more time to warn them about the penalty for disregarding the boundaries around the mountain. Entering into God's presence was (and is) a very serious matter!