Thou hast heard - You are witnesses that the prediction was uttered long before it was fulfilled.

See all this - Behold how it is all fulfilled. Bear witness that the event is as it was predicted.

And will ye not declare it? - Will you not bear witness to the entire fulfillment of the prophecy? God appeals to them as qualified to testify that what he had declared had come to pass, and calls on them to make this known as a demonstration that he alone was God (see the notes at Isaiah 44:8).

I have showed thee new things from this time - From this time I make known a thing which has not before occurred, that you may have a similar demonstration that Yahweh is God. The 'new thing' here referred to, is, doubtless, the prediction of the deliverance from the captivity at Babylon - a new thing, in contradistinction from those which had been before predicted, and which were already fulfilled (see the notes at Isaiah 42:9; Isaiah 43:19).

Even hidden things - Events which are so concealed that they could not be conjectured by any political sagacity, or by any contemplation of mere natural causes. They are, as it were, laid up in dark treasurehouses (compare Isaiah 45:3), and they can be known only by him to whom 'the darkness shineth as the day,' and to whom the night and the day are both alike Psalm 139:12.

Isaiah 48:6 Parallel Commentaries

After the Scripture.
"In the day that God created man, in the likeness of God created He him."--Gen. v. 1. In the preceding pages we have shown that the translation, "in Our image," actually means, "after Our image." To make anything in an image is no language; it is unthinkable, logically untrue. We now proceed to show how it should be translated, and give our reason for it. We begin with citing some passages from the Old Testament in which occurs the preposition "B" which, in Gen. i. 27, stands before image, where …
Abraham Kuyper—The Work of the Holy Spirit