Exodus 19:18

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.

The Lord descended upon it in fire. This was an extraordinary, unprecedented display of the Shechinah, that fiery effulgence surrounded with dark clouds, in which Yahweh is represented as appearing. On this occasion the transcendent brightness of this glory is described, in the sublime poetry of Habakkuk (Habakkuk 3:3), as covering the whole firmament far and wide above the Arabian desert.

And the whole mount quaked greatly. Since 'the shaking of the earth' is a common figure of the prophets to indicate great moral and political revolutions, the tremulous motion of Sinai was emblematic of the change which then occurred, when God took the nation of Israel into covenant, avouching Himself to be their God, and adopting them to be His special people (Deuteronomy 4:32). It was a new dispensation of Providence, to be productive in later ages of mighty moral changes on the world; and the majestic presence of Him who introduced this economy "shook," says Habakkuk poetically, "the whole earth." The author of the Epistle to the Hebrews refers the shaking of Sinai to Christ (Exodus 12:26).