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It has nothing to do with Gentile believers or the Church because Gentile believers and the Church are not the "children of promise". That title belongs to the descendants of Abraham and Isaac. The true Israel is the assembly of Jewish believers.

You just cited this, but failed to get it. "They that are children of the flesh, these are NOT the children of God: but the children of the promise are counted for the seed."

"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." (Galatians 3:16)

"For ye are all children of God by faith in Christ Jesus. For as many of you as have been baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither bond nor free, there is neither male or female: for ye are all one in Christ Jesus. And if ye be Christ's, the are ye Abraham's seed, and heirs according to the promise". (Galatians 3:26-29).