Isaac was the promised seed, Genesis 21:12 Romans 9:7: the apostle tells the Galatians that the believing Gentiles were (as Isaac) the children of the promise. Isaac being born, not by virtue of any procreative virtue in his parents, which was now dead in them, Romans 4:19, but by virtue of the promise, and by a power above nature, was a type of the believing Gentiles, who are a spiritual seed, and that seed to whom the promise was made, being the members of Christ by faith: so as the Jews had no reason so much to glory as they did, that Abraham was their father, for those amongst them that believed not were but his carnal seed, believers only were the spiritual seed, the children of the promise; to which the believing Gentiles had the same claim with the believing Jews, and a much better than those of them that believed not in Christ.
Related Commentaries of Galatians 4:28
Galatians 4:28
28 Now we, brethren, as Isaac was, are the children of promise.