Genesis 25:5

What did Abraham mean by marrying Keturah after Sarah’s death? Far be it from us to suspect him of incontinence, especially when he had reached such an age and such sanctity of faith. Or was he still seeking to beget children, though he held fast, with most approved faith, the promise of God that his children should be multiplied out of Isaac as the stars of heaven and the dust of the earth? And yet, if Hagar and Ishmael, as the apostle teaches us, signified the carnal people of the old covenant, why may not Keturah and her sons also signify the carnal people who think they belong to the new covenant? For both are called both the wives and the concubines of Abraham; but Sarah is never called a concubine (but only a wife). For when Hagar is given to Abraham, it is written. …

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. …