All ye inhabitants of the world. From exhorting the messengers to hasten on their errand, Isaiah turns to the nations generally, and bids them attend to a coming signal—an ensign is about to be raised, a trumpet is about to be sounded—let them gaze and hearken; the result will be well worth noting.
The imagery is not to be taken literally, but in the same way as the notices in Isaiah 11:10, Isaiah 11:12; Isaiah 13:2. When he lifteth up an ensign … when he bloweth a trumpet; rather, when an ensign is lifted up … when a trumpet sounds.
On the mountains. Wherever the great event took place, the signal for it was given on the mountains of Judea (see 2 Kings 19:20-34).