We must understand the OT by the revelation of the NT.
When people read this:
7 And I will establish My covenant between Me and you and your descendants after you in their generations, for an everlasting covenant, to be God to you and your descendants after you. 8 Also I give to you and your descendants after you the land in which you are a stranger, all the land of Canaan, as an everlasting possession; and I will be their God.”
They think God is making a covenant between Abraham and his natural offspring. Here "descendants" can be written "seed".
Yet, in Galatians, Paul (a Jew) reveals the matter completely:
"Now to Abraham and his Seed were the promises made. He does not say, “And to seeds,” as of many, but as of one, “And to your Seed,” who is Christ."
We can say "I have a bag of seeds" and we can also say, about the same bag, "I have a bag of seed". The former relates to the item (seeds) the later relates to its nature (seed intended to grow into something with life). Paul wrote that the second meaning (seed) is what God meant. That Seed IS Christ. There can be no mistake.
Furthermore, the writer of Hebrews reveals the nature of the covenant and its meaning:
"For when God made a promise to Abraham, because He could swear by no one greater, He swore by Himself, 14 saying, “Surely blessing I will bless you, and multiplying I will multiply you.” 15 And so, after he had patiently endured, he obtained the promise. 16 For men indeed swear by the greater, and an oath for confirmation is for them an end of all dispute. 17 Thus God, determining to show more abundantly to the heirs of promise the immutability of His counsel, confirmed it by an oath, 18 that by two immutable things, in which it is impossible for God to lie, we might have strong consolation, who have fled for refuge to lay hold of the hope set before us.
19 This hope we have as an anchor of the soul, both sure and steadfast, and which enters the Presence behind the veil, 20 where the forerunner has entered for us, even Jesus, having become High Priest forever according to the order of Melchizedek."
Two things: notice that God swore to Himself. This was a covenant God that Father to God the Son. Abraham was not held to account. God would see it through. THIS is the substance of the anchor of our soul: because God cannot be denied. There is no way this covenant can fail on account of the weakness of man.
Secondly, we see only God walking between the animals in Genesis 15. Abraham was not required to walk between them: the implication of this sign was that whomever brokered the agreement, they each agree that if either one does not see it through, the other may do to them what was done to the animals. Abraham was not required to walk between the sacrifices. God, alone, was responsible for fulfilling the covenant.
Some may say, "But the promises were given to Abraham and his seeds and only other parts of the covenant were given to Christ." If that is your conclusion then you must include the descendants of Ismael in the promises as they are also Abraham's seed. But that is not the case: the promises were given to Abraham and his Seed (singular) was Christ.
And further: "what about the land the Jews were given?" The land was given to them, true. But this was a natural event intending to show the spiritual reality. There much that I could teach about what each of these names mean as they relate to the spiritual reality of receiving the promises in Christ: "To your descendants I have given this land, from the river of Egypt to the great river, the River Euphrates— 19 the Kenites, the Kenezzites, the Kadmonites, 20 the Hittites, the Perizzites, the Rephaim, 21 the Amorites, the Canaanites, the Girgashites, and the Jebusites"
Further food for thought: Jesus the man died as a Jew. Christ arose. Christ is not a Jew, He is a many-membered man whose essence includes all men...All who look to Him receive the promises. In Him all become co-heirs with Him and they receive the promises given to Abraham. This is a spiritual transaction and is in no way related to our natural DNA.
Some may cry "Replacement theology!" (By the way, the liberals have nothing on the religious when it comes to cancel-culture. The religious have spilled the blood of Righteous Abel and the blood of the saints even to this day. Just saying.) No one is replacing the Jews. The Jews were protected. Jesus, the Seed, was born. And the sacrifice for all men, so that men could be reconciled to God, was provided. The purpose of the Jews has been fulfilled. Certainly, all of Israel will be saved but do not make the mistake of the religious: the scriptures also make it clear: "All of Israel is not Israel." Israel literally means "Prince of God". Only those in Christ are Israel. The way into Him is provided freely to the Jews and to the non-Jews.