John Calvin Commentary Isaiah 48:6

"Thou hast heard it; behold all this; and ye, will ye not declare it? I have showed thee new things from this time, even hidden things, which thou hast not known." — Isaiah 48:6 (ASV)

You have heard. This makes it still more clear that the Prophet speaks of a future captivity and of the redemption by which it would be followed; thus intending to make provision for the advantage both of the people of his own time and of posterity, so that if those who then lived received no benefit, at least posterity might take warning and repent. It frequently happens that the doctrine of the prophets produces little effect on those who see and hear, and sometimes is even treated by them with contempt, while posterity receives it with better dispositions.

See all. When he commands them to see, some think that the Prophet exhibits the event in such a manner, as if he had said that God had spoken nothing which had not been made evident to be true. But I give a different interpretation of this word חזה, (chazeh), see, in this manner: “Seeing that the Lord has spoken, it is your duty to examine the words and to pay attention.” Hence we should observe that it is owing to our sluggishness or indolence that we immediately lose those things which proceed from the mouth of God, and that many people in vain offer the pretext of ignorance, because the Lord reveals clearly enough all that is necessary to be known, if those who hear would examine it carefully, and with due attention.

And will you not declare it? The Lord next demands something more from his people than to understand and consider his word; that is, that they should be heralds and witnesses of the miracles which they have known by experience. And, indeed, he instructs his people on this condition, that they should afterwards lead others to the same confession of faith. Even now I have made you hear As if he had said, “Observe this day, on which the Lord foretells to you, by my mouth, those things which you did not know; for these things cannot be perceived or foreseen by human conjecture.”