William Burkitt on Galatians 3:29

That is, "If ye be Christ's servants and subjects, then are ye the true seed of faithful Abraham, and heirs of the blessing, according to the promise made to him and to his seed." This our apostle asserts, in opposition to the false apostles, who maintained, that there could be none tryly reputed Abraham's seed, except they were circumcised, and subjected themselves to the law of Moses: "Yes, says the apostle, if ye be Christ's, and by baptism ingrafted into him, you are the true children of Abraham, though ye be not circumcised; yea, you are heirs apparent of the heavenly inheritance given unto Abraham by promise."

Learn hence, that all sincere and serious Christians are Abraham's spiritual seed, children of his faith, though not of his flesh; and being the children of his faith, are heirs together with him of the same promises. If ye be Christ's, that is, sincere and serious Christians, then are ye Abraham's seed, and heirs according to the promise.