Notice: Forever. I stressed earlier that Abraham understood that God’s promises to him were eternal, not a historical flash-in-the-pan for one generation, his own. That is why he did not want Isaac traipsing back to Mesopotamia. Stay away from that place.
This does not mean that Abram inherited the land at that moment. God clarifies through Stephen’s comments in Acts 7.
Significantly, by giving Isaac the same promises he gave Abraham, God gave both patriarchs the same prophetic vision. Paul, in Galatians 3:8, personifies the noun “Scripture,” saying that it, the Word of God, “preached the gospel to Abraham beforehand.” Paul then tells how it did that: Abraham heard the gospel through the promises. Continuing in verse 8, Paul uses as an example the promise that all nations would receive blessings through Abraham. That blessing is most specifically a reference to the saving work of Christ, which would extend to the Gentiles.
So, both Abraham and Isaac heard the same gospel, the same promises. God inspired and motivated both men by means of the same prophetic vision. Of course, it is precisely the same prophetic vision—the same promises—that inspires and motivates us today. It is the gospel of the Kingdom of God.
Verse 15 is certainly an indication that the promise involved eternal life.