25 So then, as we are not to deny that this is the gift of God, we are thus to understand that there be some gifts of God possessed by the sons of that Jerusalem which is above, and free, and mother of us all, (for these are in some sort the hereditary possessions in which we are “heirs of God and joint-heirs with Christ:”) but some other which may be received even by the sons of concubines to whom carnal Jews and schismatics or heretics are compared. For though it be written, “Cast out the bondmaid and her son, for the son of the bondmaid shall not be heir with my son Isaac:” and though God said to Abraham, “In Isaac shall your seed be called:” which the Apostle has so interpreted as to say, “That is, not they which be sons of the flesh, these be the sons of God; but the sons of the promise are counted for the seed;” that we might understand the seed of Abraham in regard of Christ to pertain by reason of Christ to the sons of God, who are Christ's body and members, that is to say, the Church of God, one, true, very-begotten, catholic, holding the godly faith; not the faith which works through elation or fear, but “which works by love;” nevertheless, even the sons of the concubines, when Abraham sent them away from his son Isaac, he did not omit to bestow upon them some gifts, that they might not be left in every way empty, but not that they should be held as heirs. For so we read: “And Abraham gave all his estate unto Isaac; and to the sons of his concubines gave Abraham gifts, and sent them away from his son Isaac.” If then we be sons of Jerusalem the free, let us understand that other be the gifts of them which are put out of the inheritance, other the gifts of them which be heirs. For these be the heirs, to whom is said, “You have not received the spirit of bondage again to fear, but you have received the spirit of adoption of sons, whereby we cry, Abba, Father.”
Source: On Patience (New Advent)