It is important to emphasize that when verse 45 talks about all the Lord's good promises being fulfilled, that really does mean that the promise made to Abraham has been realized, at least in the sense of a first fulfillment. Israel is now in the land; the physical descendants of Abraham are living in Canaan. God’s promise has been kept.
For us today, this truth has relevance especially when it comes to the modern nation-state of Israel. The fact that there are now Jews living in Jerusalem and the surrounding areas is not a fulfillment of the promise made to Abraham. The promise about descendants living in the land of Canaan was fulfilled, as said in Joshua 21:45. Israel's settling in Canaan is the moment in time when that promise came to completion. We must not, therefore, think that it is our job as Christians to support and advance the interests of modern-day Israel. There is nothing holy about that land and there are also not two peoples of God, Jews and Christians (see Ephesians 2:1–22). No, the modern Israel is not the Israel that lived under the old covenant.
We can know this with certainty because of what we read in the New Testament. With the coming of Christ, this promise about land is radically transformed. And it is transformed in two ways.
Firstly, it is interpreted as a reference to the new creation. In the book of Hebrews, the writer tells us that there is a rest which remains for the people of God, a rest which was not fully achieved in the time of Joshua (Hebrews 4:1–16). He then goes on to tell us in Hebrews 11:8–16 that Abraham himself, the man who received the original promise from God, understood that promise as ultimately referring to the new creation. He saw the land of Canaan with his eyes, he touched it with his hand, he buried his wife in one of its caves—yet as he did so, he recognized that this land was but a foreshadowing of the land to come. And so he longed for the city where there is no death, the city that has eternal foundations, the city whose designer and builder is God.
A second way in which the promise is transformed has to do with the time in which we are now living. In Romans 4:13, Paul mentions that the promise to Abraham was that he and his offspring would be heirs to the world. Through him and from his descendants would come a blessing to all the nations (Genesis 12:3). He was thus not simply a title holder in the land of Canaan, but heir to the whole world. After the coming of Christ, not only the land of Canaan but the whole world will be filled with Abraham’s seed (see Galatians 3:14, Galatians 3:26–29). From every nation there would be those who, in the words of Psalm 87:1–7, are born in Zion.
Individuals, families, and communities will hear the message of Christ and be reconciled to him.
The promise of land then is not revoked but transformed with the coming of Christ. Before his coming, this promise was fulfilled in the land of Canaan, but after his coming it refers to the whole world and especially the world to come. Christian believers fill this world now and one day we will also fill the new creation. The modern nation-state of Israel is not the fulfillment of this promise to Abraham. Instead, this promise is being fulfilled in those who believe in Jesus Christ. It is being fulfilled in the fact that Jews who believe in Jesus Christ, and you and I—gentile believers with white and black skin and no Jewish ancestry—are now all God's people. We are now all called to serve him in this world (Colossians 1:13).
43 Thus the LORD gave to Israel all the land that he swore to give to their fathers. And they took possession of it, and they settled there.