The context of Isaiah 40 is a new beginning for Israel. Judah, the remaining two tribes, had spent seventy years in captivity in Babylon. God brought them out of exile and back to the land. However—and this is frequently overlooked—the other ten tribes never emerged from exile. They were lost, scattered among the disinherited nations. But the coming of the messiah will result in redemption for all the tribes. Yahweh will draw his children from every tribe and nation, whether Abraham’s literal descendants or not.
