Moses first met God near Sinai
Before Israel arrived at Sinai as a people, Moses encountered God there while caring for sheep. He saw a bush burning without being destroyed, and God called to him from it. The Lord then sent Moses back to Egypt to bring the Israelites out of slavery (Exodus 3:1-5; Exodus 3:10).
Exodus calls this place Horeb in that account, while the later wilderness story calls it Sinai. Christians and Jewish readers commonly understand Horeb and Sinai as names for the same mountain, or for the same mountain region. Either way, this was the place where God began preparing Moses for the work ahead.
Now Moses kept the flock of Jethro his father in law, the priest of Midian: and he led the flock to the backside of the desert, and came to the mountain of God, even to Horeb. And the angel of the Lord appeared unto him in a flame of fire out of the midst of a bush: and he looked, and, behold, the bush burned with fire, and the bush was not consumed. And Moses said, I will now turn aside, and see this great sight, why the bush is not burnt. And when the Lord saw that he turned aside to see, God called unto him out of the midst of the bush, and said, Moses, Moses. And he said, Here am I. And he said, Draw not nigh hither: put off thy shoes from off thy feet, for the place whereon thou standest is holy ground.
Israel entered a covenant with God
Around three months after leaving Egypt, Israel camped before Mount Sinai. God reminded them that he had carried them out of Egypt, then invited them into a covenant, a committed relationship in which they would belong to him and listen to his voice (Exodus 19:1-6). The people agreed together to do what God had spoken (Exodus 19:7-8).
The mountain was wrapped in thunder, lightning, thick cloud, trumpet sound, smoke, and fire. It shook as God came down upon it, while Moses went up to meet him (Exodus 19:16-20). These signs showed God's holiness. He was close to his people, but he was never ordinary or safe to treat casually.
And it came to pass on the third day in the morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of the trumpet exceeding loud; so that all the people that was in the camp trembled. And Moses brought forth the people out of the camp to meet with God; and they stood at the nether part of the mount. And mount Sinai was altogether on a smoke, because the Lord descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly. And when the voice of the trumpet sounded long, and waxed louder and louder, Moses spake, and God answered him by a voice. And the Lord came down upon mount Sinai, on the top of the mount: and the Lord called Moses up to the top of the mount; and Moses went up.
God gave his law and instructions for worship
At Sinai God gave Israel the Ten Commandments. They called Israel to worship God alone, reject idols, honour his name and Sabbath, respect parents, and live truthfully and faithfully with one another (Exodus 20:1-5; Exodus 20:8-12; Exodus 20:13-17). God had already rescued them from Egypt. The law was their response to grace, not a way to earn it.
Moses read the book of the covenant to the people, and the covenant was confirmed (Exodus 24:7-8). He then remained on the mountain for forty days and nights, where he received the stone tablets and instructions for the tabernacle, the portable sanctuary where God would dwell among Israel (Exodus 24:18; Exodus 25:8-9; Exodus 31:18).
And he took the book of the covenant, and read in the audience of the people: and they said, All that the Lord hath said will we do, and be obedient. And Moses took the blood, and sprinkled it on the people, and said, Behold the blood of the covenant, which the Lord hath made with you concerning all these words.
The golden calf brought rebellion and renewal
While Moses remained on the mountain, the people became impatient and asked Aaron for visible gods. Aaron made a golden calf, even though Israel had just committed itself to the God who had delivered them from Egypt (Exodus 32:1-4). When Moses saw the scene, he broke the stone tablets and confronted their sin (Exodus 32:19-20).
Yet failure was not the final word at Sinai. Moses pleaded for the people, and God told him to prepare new tablets. The covenant was set down again, showing both the seriousness of idolatry and God's mercy toward his undeserving people (Exodus 34:1-4; Exodus 34:27-28).
Sinai matters because it holds these truths together: God saves, God is holy, God calls his people to faithful living, and God does not abandon them lightly when they fail. Reading these scenes slowly can help make the story of Israel, and God's character within it, much clearer.
And the Lord said unto Moses, Write thou these words: for after the tenor of these words I have made a covenant with thee and with Israel. And he was there with the Lord forty days and forty nights; he did neither eat bread, nor drink water. And he wrote upon the tables the words of the covenant, the ten commandments.