John Gill 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, see all this: and will you not declare it ?
&c.] You have heard of all these things, how they were foretold before they were; how they came to pass exactly as they were predicted. Now look over these prophecies, and compare them with the events; see the exact completion of them. And when you have so done, can you be so stouthearted and impudent as to deny them, or not own and confess them?

I have showed you new things from this time, even hidden things, and
you did not know them ;
meaning the destruction of the Babylonish empire, and the deliverance of the Jews by Cyrus, prophesied just now in the preceding chapters; things not yet come to pass, newly revealed, which were hidden in the breast of God, and unknown to them until prophesied of.

And which were typical of redemption by the incarnate Son of God, whose incarnation, and salvation by him, were new, unheard of, and wonderful things.

And of the new state of things under the Gospel dispensation, when all things shall become new; the doctrines and ordinances of which are new; the whole Gospel is a hidden mystery, and unknown to men till revealed and made known by the Spirit of God.