John Calvin Commentary Numbers 20:8

"Take the rod, and assemble the congregation, thou, and Aaron thy brother, and speak ye unto the rock before their eyes, that it give forth its water; and thou shalt bring forth to them water out of the rock; so thou shalt give the congregation and their cattle drink." — Numbers 20:8 (ASV)

Take the rod. It is unquestionable that the faith of Moses had now begun to waver; but we gather from his prompt obedience that it had not altogether failed; for he wastes no time in discussion, but comes straight to the rock to perform God’s command. His faith, then, was only so smothered, that its hidden strength at once directed him to his duty. Thus, the saints sometimes, while they totter like children, still advance toward their mark.

By the sight of “the rod,” God would recall to both Moses and the people so many miracles, which were well fitted to awaken confidence for the future; just as if He were uplifting the standard of His power. The command to speak to the rock is not without a severe reproach, as if He had said that in the lifeless elements there was more reason and intelligence than in people themselves. And assuredly, it was something to be greatly ashamed of that the rock, as if it could hear and was endowed with sense, should obey God’s voice, while the people, to whom the Law had been given, remained in deafness and stupidity.