John Calvin Commentary Exodus 19:21

"And Jehovah said unto Moses, Go down, charge the people, lest they break through unto Jehovah to gaze, and many of them perish." — Exodus 19:21 (ASV)

And the Lord said to Moses. By God’s command the same prohibition is repeated: that the people should not cross the boundaries. Undoubtedly, forbidding them once was not enough, as we can gather from Moses' reply, for he thought that since they were all admonished, there was no necessity for a new prohibition.

But God insists more vehemently and, again with threats, orders them to be charged to take diligent heed to themselves. Indeed, He knew that He was dealing with rebellious people, for whose subjugation a greater fear of punishment would be necessary.

Now, since we are no better than they, let us not be surprised if God often spurs us on with many exhortations and redoubles His threats; otherwise, forgetfulness of all that He has once commanded would creep over us.

This passage also confirms the fact that the curiosity which influences people's minds is very displeasing to God. For He expressly commands that they should not break through to gaze—not because He wishes to conceal or hide anything it would be profitable for them to know, but because their inquiries should be sober. This is the proper limit of knowledge: to humbly learn from God's mouth what He voluntarily teaches—not to push forward with overly anxious desires, but to follow Him as He leads us.