That is answered in the Bible. The Abrahamic Covenant Is Unconditional and Eternal
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for the sake of the fathers" comes from the Apostle Paul in
Romans 11:28. It refers to the enduring love God has for the Jewish people—the original branches of His covenant tree—because of His ancient, irrevocable promises to their founding ancestors (the patriarchs Abraham, Isaac, and Jacob). This biblical teaching highlights several key principles regarding God’s promises: Unconditional Love: Paul explains that even though the Israelites rejected the gospel at the time, they remain deeply beloved by God not for their own present actions, but on account of their forefathers.
Romans 11:4 But what saith the answer of God unto him? I have reserved to myself seven thousand men, who have not bowed the knee to the image of Baal.
5 Even so then at this present time also there is a remnant according to the election of grace.
Paul teaches the Romans that the Gospel must continue to be offered to hid kinsmen.
The reason, because God intends to save a REMNANT of them.
The Romans do not know who these Jews will be, but God does because He "foreknew"(v.2) them.
These Jews are going to be Christians.
They are going to be citizens of the "Israel of God" Galatians 6:15 and 16.
They will also be counted as "Abraham's seed, and heirs according to the promise" Galatians 3:29.
The Abrahamic Covenant was made to Christ.
Galatians 3:16
Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Notice that the believers from the other nations are included in all these blessings too.
That is what Paul refers to in Romans 11:26 when he says "And so all Israel shall be saved".