Galatians 4:28

We were born not according to nature but according to grace. For, just as in Isaac's case, it was not the law of nature but that of the gospel that fashioned us. Thus the promise given to Abraham engendered us.

For not alone upon Abraham's account did He say these things, but also that He might point out how all who have known God from the beginning, and have foretold the advent of Christ, have received the revelation from the Son Himself; who also in the last times was made visible and passable, and spake with the human race, that He might from the stones raise up children unto Abraham, and fulfil the promise which God had given him, and that He might make his seed as the stars of heaven, as John the Baptist says: "For God is able from these stones to raise up children unto Abraham." Now, this Jesus did by drawing us off from the religion of stones, and bringing us over from hard and fruitless cogitations, and establishing in us a faith like to Abraham. As Paul does also testify, saying that we are children of Abraham because of the similarity of our faith, and the promise of inheritance.

If, then, God promised him the inheritance of the land, yet he did not receive it during all the time of his sojourn there, it must be, that together with his seed, that is, those who fear God and believe in Him, he shall receive it at the resurrection of the just. For his seed is the Church, which receives the adoption to God through the Lord, as John the Baptist said: "For God is able from the stones to raise up children to Abraham." Thus also the apostle says in the Epistle to the Galatians: "But ye, brethren, as Isaac was, are the children of the promise." And again, in the same Epistle, he plainly declares that they who have believed in Christ do receive Christ, the promise to Abraham thus saying, "The promises were spoken to Abraham, and to his seed. Now He does not say, And of seeds, as if [He spake] of many, but as of one, And to thy seed, which is Christ." And again, confirming his former words, he says, "Even as Abraham believed God, and it was accounted to him for righteousness. Know ye therefore, that they which are of faith are the children of Abraham. But the Scripture, fore-seeing that God would justify the heathen through faith, declared to Abraham beforehand, That in thee shall all nations be blessed. So then they which are of faith shall be blessed with faithful Abraham." Thus, then, they who are of faith shall be blessed with faithful Abraham, and these are the children of Abraham. Now God made promise of the earth to Abraham and his seed; yet neither Abraham nor his seed, that is, those who are justified by faith, do now receive any inheritance in it; but they shall receive it at the resurrection of the just. For God is true and faithful; and on this account He said, "Blessed are the meek, for they shall inherit the earth."

One might ask how he speaks of the Galatians, whom he had called fools. He accused them of starting in the Spirit and finishing in the flesh. When the apostle called them "sons of promise" in the way that Isaac was, he meant that he did not completely despair of their salvation and judged that they would return again to the Spirit, in which they had begun, and become sons of the free woman.

It is not merely that the Church was barren like Sarah, or became a mother of many children like her, but she bore them in the way Sarah did. As it was not nature but the promise of God which rendered Sarah a mother, [for the word of God which said, "At the time appointed I will return unto thee, and Sarah shall have a son," (Gen. 18:14.) this entered into the womb and formed the babe,] so also in our regeneration it is not nature, but the Words of God spoken by the Priest, (the faithful know them,) which in the Bath of water as in a sort of womb, form and regenerate him who is baptized. Wherefore if we are sons of the barren woman, then are we free. But what kind of freedom, it might be objected, is this, when the Jews seize and scourge the believers, and those who have this pretence of liberty are persecuted? for these things then occurred, in the persecution of the faithful. Neither let this disturb you, he replies, this also is anticipated in the type, for Isaac, who was free, was persecuted by Ishmael the bondman. ...