Romans 9:8

The children of the flesh; Abraham's children by mere natural descent.

The children of God; in a special sense, as being heirs of the promises made to Abraham.

The children of the promise; the children of Isaac, who was born in a supernatural way, according to God's promise, mentioned in the following verse.

Are counted for the seed; the seed that inherits the promises made to Abraham. Thus the apostle shows that from the very beginning the blessings of the covenant did not flow in the line of mere natural descent to Abraham's children, but according to God's promise. Upon the same principle God acts under the gospel, reckoning as the true seed of Abraham those, and those only, who are born, not of blood, nor of the will of man, but of God, and who show this by believing in Christ. John 1:13; John 6:29. The promises of the gospel are not made to any on account of their natural descent or their religious privileges, but on account of their union to Christ by believing on him.