John Calvin Commentary Exodus 19:9

"And Jehovah said unto Moses, Lo, I come unto thee in a thick cloud, that the people may hear when I speak with thee, and may also believe thee for ever. And Moses told the words of the people unto Jehovah." — Exodus 19:9 (ASV)

And the Lord said to Moses. God here proclaims that, by a manifest symbol of His glory, He will make it evident that the Law did not proceed from Moses, but that he faithfully delivered only what he received from heaven. For God was so covered with the cloud, as with a veil, that He still uplifted their minds by a clear sign of His presence.

On this, the authority of Moses was founded: that the Israelites knew God to be the author of the doctrine, of which Moses was the minister. This is especially worth noting, because we gather from this that there is no other way of proving a doctrine except by the assurance that it comes not from anywhere else, but from God alone.

Thus, every mortal is brought down to his proper level, so that no one, however excellent in wisdom, should dare to advance his own imaginations. For if Moses, the mightiest of prophets, obtained credit in the Church on no other grounds than because he bore the commands of God and taught only what he had heard, how foolish and impudent it will be for teachers, who are far beneath him, to endeavor to attain a higher point!

In summary, this passage shows that we must believe in God alone, but at the same time, we must listen to the prophets who spoke from His mouth. Besides this, it appears that God did not wish His servant Moses to gain credit only for a short time, but that posterity should pay him the same reverence even after his death.

The call of some is temporary, and it may happen that God takes away the spirit of prophecy from those to whom He has given it. But He appeared to Moses in such a way as to ratify and, as it were, consecrate the truth of his doctrine for all ages. From this it follows that the brightness of God’s glory, which was shown to His ancient people in the thick cloud, is not yet extinct.

Instead, it ought to illuminate the minds of all the godly, so they may reverently submit themselves to Moses. What follows at the end of the verse is a repetition from the previous one, for there was no intervening reply from the people that Moses could report. The meaning is that although the Israelites had voluntarily promised to remain in the path of duty, this confirmation was still added, like a spur to those who are running, so that they might proceed more quickly.