Then again Abraham took a wife, and her name was Keturah.
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Augustine of Hippo
As for those who prefer to read no symbolic meanings into such facts, they still have no ground of complaint against Abraham. For, in the literal sense, there may be meant to be here an argument against those heretics who are opposed to second marriages, since the example of the very father of many nations proves that there is no sin in a second marriage that is made after one’s wife is dead.
Because no law concerning virginity or chastity had been set down, lest desire ever make a stain in the mind of that just man …. Abraham took for himself a concubine after the death of Sarah, so that through the uprightness of his many sons who were to be scattered throughout the entire earth, knowledge and worship of the one God would be spread. Abraham then had sons from Keturah, and he sent them eastward with gifts. Abraham died years old and was buried next to Sarah, his wife.
Cetura, his third wife; the former two being perhaps both dead. This Abraham did in his 137th year, that God might have witnesses also among the Gentiles. Cetura was before one of his handmaids. (Menochius)
God enabled him to have children at this advanced age; or perhaps, Moses may have related his marriage in this place, though it had taken place several years before. (St. Augustine, contra Jul. iii.) (Calmet) This learned father, City of God xvi. 34, supposes that the reason why Cetura is styled a concubine, though she was a lawful and only wife, is because her children prefigured heretics, who do not belong to the kingdom of Christ. (Worthington)