Jamieson, Fausset and Brown on Isaiah 48:6

6. Thou, &c.--So "ye are my witnesses" (Is 43:10). Thou canst testify the prediction was uttered long before the fulfilment: "see all this," namely, that the event answers to the prophecy.

declare--make the fact known as a proof that Jehovah alone is God (Is 44:8).

new things--namely, the deliverance from Babylon by Cyrus, new in contradistinction from former predictions that had been fulfilled (Is 42:9; 43:19). Antitypically, the prophecy has in view the "new things" of the gospel treasury (So 7:13; Mt 13:52; 2Co 5:17; Re 21:5). From this point forward, the prophecies as to Messiah's first and second advents and the restoration of Israel, have a new circumstantial distinctness, such as did not characterize the previous ones, even of Isaiah. Babylon, in this view, answers to the mystical Babylon of Revelation.

hidden--which could not have been guessed by political sagacity (Da 2:22, 29; 1Co 2:9, 10).