This is a direct promise of not only Abraham's children's owning of the land, but also of Abraham's personal ownership of it. Yet the only land he ever truly owned was Sarah's burial plot—certainly not all the land he could see! For him to receive this promise, and for him to receive it “forever,” means that he and his descendants will live forever.
Now eternal life has entered the picture. Eternal life includes a spirit body that will not decay and a nature that is appropriate or fitting for endless life, one that is sinless and not continually incurring the death penalty. Only in the resurrection of the dead at Christ's return will the called of God—including Abraham—be raised incorruptible and given immortality, such that death is swallowed up in victory (see I Corinthians 15:42-54). Then, Abraham and his spiritual descendants will inherit the Promised Land, retaining it forever.
Romans 4:13 expounds on the promise of the land: “For the promise that he would be the heir of the world was not to Abraham or to his seed through the law, but through the righteousness of faith.” God's promise to Abraham was not based on perfect obedience to the law, but on the imputed righteousness that comes by faith, which happened when Abraham “believed in the LORD, and He accounted it to him for righteousness” (Genesis 15:6). This took place well before the covenant of circumcision (Genesis 17:1-14). Abraham's faith produced good works, as true faith always will; in Genesis 26:5, God says, “Abraham obeyed My voice and kept My charge, My commandments, My statutes, and My laws.” But his righteousness in God's eyes was shown in his belief in God's faithfulness, not in anything he did or did not do.
Clearly, God's promise of the land to Abraham goes far beyond physical inheritance—it is, rather, an eternal inheritance, bestowed on those who have become his spiritual descendants through receiving the faith of Abraham. The patriarch, though, was among those who “died in faith, not having received the promises, but having seen them afar off were assured of them” (Hebrews 11:13). So significant are these promises that God confirmed them with a covenant that condemned Him to destruction if He failed to fulfill the terms. Not only that, the timing of Christ's sacrifice coincided with the preparations for God's covenant with Abraham, for it is His sacrifice that allows us—Abraham's spiritual seed, his “great nation”—to begin to receive these promises.
David C. Grabbe
Why Was Jesus Not Crucified as Passover Began? (Part Two)