John Calvin Commentary Exodus 19:16

"And it came to pass on the third day, when it was morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of a trumpet exceeding loud; and all the people that were in the camp trembled." — Exodus 19:16 (ASV)

And it came to pass on the third day. We must bear in mind what I have already referred to: this terrible spectacle served partly to set the presence of God before their eyes, so that His majesty might urge the beholders to obedience and defend His doctrine against contempt, and partly to express the nature of the Law, which in itself produces nothing but terror.

The air was disturbed by thunder and lightning, and the sound of the trumpet; the mountain was wrapped in smoke and darkness, so that the people might humbly prostrate themselves before God and solemnly embrace the covenant proposed to them. For religion never penetrates the mind to the point that it seriously receives God’s word until its vices are cleansed and corrected, and it is truly subdued.

This fear is also common to the Gospel. For just as God shook the earth in the promulgation of the Law, so also when He speaks by the Prophet about the coming of Christ and the restoration of His Church, He says, Yet once, it is a little while, and I will shake the heavens and the earth, etc. (Haggai 2:6). In the same way, David, when he wishes to portray God as the avenger of His Church, describes Him using this imagery. For undoubtedly, when he says in Psalm 18:7-9, Then the earth shook and trembled, the foundations also of the hills moved and were shaken — there went up a smoke out of his nostrils, and fire out of his mouth devoured; he bowed the heavens also, and came down; and darkness was under his feet, he alludes to the history that Moses relates here. Habakkuk 3:3 does so even more plainly — God came from Teman, and the Holy One from Mount Paran.

Meanwhile, the other point remains: the awe-inspiring wonders, at which the people inevitably trembled, were added as seals to the promulgation of the Law. This was because the Law was given to summon slumbering consciences to the judgment seat, so that, through fear of eternal death, they might flee for refuge to God’s mercy.