Old Covenant Israel was bred in captivity. They spent four-hundred years serving as slaves to the Egyptians before God brought them out with signs and wonders. But Israel didn't stay free once they were liberated. God's Old Covenant people were a particularly disobedient lot, and so, after much warning, the Lord sent them into captivity again - this time in two different directions: the Northern Kingdom of Israel to Assyria and the Southern Kingdom of Judah to Babylon. Prophecy abounded with promises that one day the Lord would bring His people back from the lands where they had been scattered, and He was faithful to His gracious word. He brought some back to rebuild what had been destroyed, but it was only a remnant - far smaller than the millions that had crossed the Red Sea. A people that was once counted by everyone born of the lineage of Jacob was now being counted as only those who had the faith to return.
But there were other gracious and glorious truths hidden by mystery in the words of the prophets that promised a return. The original promise to Abraham to make his descendants as numerous as the stars or the sand on the seashore was not slowly dwindling, it was ready to break forth in all its fullness.
In the eleventh chapter of Isaiah, we first read of a "shoot from the stump of Jesse", a promised King that would be anointed with the Spirit, that would be clothed in righteousness and justice, and that would bring peace. A Messiah ("anointed") would come to save God's people:
In that day the root of Jesse, who shall stand as a signal for the peoples—of Him shall the nations inquire, and His resting place shall be glorious. In that day the Lord will extend his hand yet a second time to recover the remnant that remains of His people, from Assyria, from Egypt, from Pathros, from Cush, from Elam, from Shinar, from Hamath, and from the coastlands of the sea. (Isaiah 11:10-11)
When the Messiah would come, the call would be sounded forth and the captives would come from all of the lands that they had been scattered, but the wondrous mystery was that these were not all descended from Jacob! His people would not only include Jews who had been exiled to lands like Assyria and Egypt and Shinar, but would now include the former enemies themselves. All those gifted with faith to trust this Messiah would answer this call, and the promise to Abraham would find fulfillment in those who shared his faith, not merely those who shared his bloodline.
A Jew reading the prophecy of Isaiah before the time of the Christ would have only seen there Jews returning from the lands that had enslaved them. But as I read it from this side of the cross, I can clearly see how God promised that the nations would inquire of this Christ, and how He is calling His people out of these nations even though they belong to those nations ethnically.
What an amazing thing: to be grafted into the people of God through faith! When God's call went out to the nations for His people to return to Him, it came also to me - and I can trace no part of my ancestry back to Abraham! We belong to God's family not because of our flesh, but because of faith - a faith which He supplies. What right have I to claim a part in God's covenant? None. He called me into it, and so I rejoice, and I broadcast the call to others.