"Those who are of faith" are those who have the same disposition of spirit that Abraham had. It is the disposition of faith, of hope against hope.
They properly constitute his promised race and are his sons according to the promise.
Countless posterity was promised to Abraham, and it came to be. But fleshly descent from Abraham alone does not make one his descendant.
What especially matters is to be of one spirit with him. Thus even fleshly descendants who do not have this spirit are not descendants, as the Lord told the Jews to their face (see Jn 8:39).
On the contrary, those who are not fleshly descendants, yet are full of this spirit, are descendants.
Saint Damascene writes: "The Gentiles who have believed should boast of Abraham as father, or as the homeland of Abraham, more than those who are of circumcision.
"Faith made Abraham a friend of God, not circumcision. Circumcision was already a seal and sign of faith.
"The sign is less than that of which it is the sign. Therefore those who are of faith ought more rightly to be called sons of Abraham than those who are of circumcision."
Jerome explains this more fully: "Here the Apostle says that those are sons of Abraham who believe in the same spirit in which uncircumcised Abraham believed.
"Therefore the Lord said to the unbelieving Jews, 'If you were Abraham's children, you would do the works of Abraham' (Jn 8:39).
"What works did the Lord expect of them when He said this, except faith? For He had already said to them before: that you believe in Him Whom God has sent (cf. Jn 6:29).
"In another place it was said to Jews who trusted in their descent and the antiquity of their race: 'Do not begin to say within yourselves, We have Abraham as our father.
"'For God is able from these stones to raise up children to Abraham' (cf. Mt 3:9).
"By stones, as no one will doubt, are signified the wild and coarse hearts of the Gentiles, which afterward were softened and received the seal of faith.
"Therefore, discerning reader, enumerate the virtues by which Abraham pleased God before circumcision.
"Whomever you find like him in this, call him a son of Abraham, who was justified in uncircumcision."