Albert Barnes Commentary Galatians 3:29

"And if ye are Christ`s, then are ye Abraham`s seed, heirs according to promise." — Galatians 3:29 (ASV)

And if you are Christ's. If you belong to the Messiah and are interested in His work.

Then you are Abraham's seed. The promise made to Abraham related to the Messiah. It was a promise that in Him all would be blessed. Abraham believed in that Messiah and was distinguished for his faith in Him who was to come. If they believed in Christ, therefore, they showed that they were the spiritual descendants of Abraham. No matter whether they were Jews or Gentiles—whether they had been circumcised or not—they had the same spirit which he showed, and were interested in the promises made to him.

And heirs according to the promise. See Romans 8:17. You are heirs of God. You inherit the blessings promised to Abraham and partake of the blessedness to which he looked forward. You have become truly heirs of God, and this is in accordance with the promise made to Abraham.

It is not by the obedience of the law; it is by faith—in the same way that Abraham possessed the blessing: an arrangement before the giving of the law, and therefore one that may include all, whether Jews or Gentiles. All are on a level; and all are alike the children of God, in the same manner, and on the same terms as Abraham was.