You have the privilege of hindsight and thousands of years' benefit. The Abraham Covenant is recorded in two parts between chapters 12 to 17 of Genesis. Abraham wasn't thinking about spirituality when he asked the LORD for a child to pass on his possessions. All of that spirituality stuff came about later. Abraham was only asking for a physical child not a spiritual one. The first problem that he was confronted with was a child from a slave (Hagar) and one freeborn (Isaac.) But this was later in their lives. At least a decade later. Abe didn't even know what "spiritual" meant for he existed on a material plane. There was no concept to Abraham of spiritual when Ishmael was born, not even the idea of bondage to freeborn concepts.
Saul was able to reason out ideas of bondage and freeborn and physical and spiritual through study of the Hebrew Scripture (Genesis to Malachi - although the order is different in an actual Hebrew bible), He had the whole counsel of Scripture to do this for the first writings of the Hebrew Bible did not come until God set finger to stone and Moses later of pen to parchment some 400 years later on. And for over 400 years from the beginning of God's Promises to Abraham to Moses was Isaac, then Jacob, then the twelve sons of Jacob, and then to over 4 million Jews who departed Egypt led by Moses to where? Their only foresight was to a place where Moses told Pharaoh that he wanted to take them so they could worship.
2 And Pharaoh said, Who is the LORD, that I should obey his voice to let Israel go? I know not the LORD, neither will I let Israel go.
3 And they said, The God of the Hebrews hath met with us: let us go, we pray thee,
three days’ journey into the desert, and sacrifice unto the LORD our God; lest he fall upon us with pestilence, or with the sword.
Exodus 5:2–3.
That's all the Hebrews knew. Moses was taking "us" into the wilderness, and it would take us three days' worth of journeying. Israel of twelve tribes and over 4 million including children would go on for three days, stop, pitch tents and worship the LORD. Do you know what over four million Hebrews looks like from an airplane? Balaam saw it from up a high mountain in Numbers. Four million filled the Jordan valley from end to end. At this time Israel walked by sight not by faith. There was no "faith" The Hebrews trusted Moses only as far as they could throw him. But from your perspective thousands of years later you have the whole WRITTEN counsel of God and can NOW perceive the spiritual aspect of the Abraham Covenant. It was all about having a physical son to inherit Abraham's possessions and carry his name forward. There was nothing in Abraham's mind about ONE spiritual person in which all the Promises would rest. In his mind it was about one physical person in which the promises would rest: Isaac.
And God told Abraham in chapter 15 of Genesis exactly what his borders would look like. All he knew was physical borders and Isaac would be the one to manage the land. That was his expectation. How could this be realized if God told him his seed was going to be in a strange land and afflicted?
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