Charles Spurgeon Commentary Romans 9:8

"That is, it is not the children of the flesh that are children of God; but the children of the promise are reckoned for a seed." — Romans 9:8 (ASV)

That is, your flesh, which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.

Now, Isaac was not the child of Abraham's flesh. He was born according to promise, when his mother was past childbearing age, and his father was advanced in years. His was the birth according to the promise, and that is how the line of grace runs—not according to the flesh, but according to the promise.

If, then, all my hope of heaven lies in my being a child of godly parents, it is an Israelite hope, and good for nothing. If my hope of heaven lies in my having been born according to the promise of God—born of his grace and his power—then in that line the covenant stands. God is determined that it shall be so.