John Calvin Commentary Isaiah 6:9

"And he said, Go, and tell this people, Hear ye indeed, but understand not; and see ye indeed, but perceive not." — Isaiah 6:9 (ASV)

Then he said, Go, and tell that people. This shows still more clearly how necessary the vision was, so that Isaiah might not all at once fail in his course. It was a grievous stumbling block that he must endure such obstinacy and rebellion among the people of God, and that not only for a year or two, but for more than sixty years.

For this reason, he needed to be fortified, so that he might be like a brazen wall against such stubbornness. The Lord, therefore, simply forewarns Isaiah that he will have to deal with obstinate men, on whom he will produce little effect; but that such an unusual occurrence must not lead him to take offense, lose courage, or yield to the rebellion of men; that, on the contrary, he must proceed with unshaken firmness and rise superior to temptations of this nature.

For God gives him due warning beforehand about the result; as if he had said, “You will indeed teach without any good effect; but do not regret your teaching, for I enjoin it on you; and do not refrain from teaching because it yields no advantage; only obey me, and leave to my disposal all the consequences of your labors. I give you all this information in good time, so that the event may not terrify you, as if it had been strange and unexpected.”

Besides, he is commanded to reprove their blind obstinacy openly, as if he purposely taunted them.

“My labors will do no good; but it does not matter to me: it is enough that what I do meets God’s approval, to whom my preaching will be a sweet smell, even if it brings death to you” (2 Corinthians 2:15–16).