1 Then again Abraham took a wife, and her name was Keturah.
[AD 253] Origen of Alexandria on Genesis 25:1
The holy apostle always offers us opportunities for spiritual understanding and shows the zealous signs by which one may recognize in all things that “the law is spiritual.” Though few, these signs are nevertheless necessary.Paul says, discussing Abraham and Sarah in a certain passage, “not weakened in faith.” Scripture says, “He considered his own body dead, since he was almost a hundred years old, and Sarah’s womb dead.” This man, therefore, whom Paul says to have been dead in his body at the age of 100 and to have begotten Isaac more by the power of his faith than by the fertility of his body, Scripture now relates has taken a wife named Keturah and has begotten more sons from her when he seems to have been about 137 years old. For Sarah his wife is recorded to have been ten years younger than he. Since Sarah died in her 127th year, it shows that Abraham was more than 137 years old when he took Keturah as his wife.
What then? Are we to suppose that inducements of the flesh have flourished in so great a partriarch at that time? And shall he who is said to have been dead long ago in his natural impulses now be supposed to have been revived for passion? Or, as we have already often said, do the marriages of the patriarchs indicate something mystical and sacred, as also he suggests who said of wisdom: “I decided to take her as my wife”?
Perhaps, therefore, already at that time Abraham also thought something like this. And, although he was wise, for this very reason never-theless he knew that there is no end of wisdom, nor does old age impose a limit on learning. For when can that man who has been accustomed to share a marriage in that manner in which we indicated above, that is, who is accustomed to have virtue in marriage, cease from such a union? For indeed the death of Sarah is to be understood as the consummation of virtue. But a man of consummate and perfect virtue ought always to be engaged in some learning. The divine language calls this learning his wife.

[AD 253] Origen of Alexandria on Genesis 25:1
[One] who wishes to show himself to be a child of Abraham by doing the works of Abraham in accordance with the Savior’s explanation need not literally have sexual intercourse with a handmaid or take another wife in old age after the death of his wife. We also learn from this quite clearly that we must interpret the whole story of Abraham allegorically and make each thing he did spiritual, beginning with the command, “Go forth from your land, your kindred, and your father’s house, into the land that I will show you.” This statement is made not only to Abraham but also to everyone who will be his child.

[AD 253] Origen of Alexandria on Genesis 25:1
Indeed, Keturah, whom Abraham, now an old man, obtains in marriage means thymiama, which is incense or a pleasing fragrance. He also in fact was saying, just as Paul said, “We are the pleasing fragrance of Christ.” But let us see how someone becomes “Christ’s pleasing fragrance.” Sin is a foul affair. In fact, sinners are compared with pigs that wallow in sins as in foul dung. And David, as a repentant sinner, says, “My sores have putrified and are abscessed.”12If there is therefore any one of you in whom there is now no odor of sin but an odor of justice, the sweetness of mercy, if anyone, by praying “without ceasing” always offers incense to the Lord and says, “Let my prayer be directed as incense in your sight, the lifting up of my hands as the evening sacrifice,” this man has married Keturah. In this way, therefore, I think the marriages of the elders are interpreted more fittingly; in this way the unions entered by the patriarchs in their now final and weakened age are understood nobly; in this way I hold the necessary begetting of children should be reckoned. For young men are not so well fitted as old men for such marriages and for offspring of this kind. For to the extent that someone is feeble in the flesh, to such an extent will he be stronger in virtue of the soul and more fit for the embraces of wisdom.

[AD 253] Origen of Alexandria on Genesis 25:1
In this way also you can, if you wish, be a husband of marriages of this kind. For example, if you freely practice hospitality, you will appear to have taken her as your wife. If you shall add to this care of the poor, you will appear to have obtained a second wife. But if you should also join patience to yourself and gentleness and the other virtues, you will appear to have taken as many wives as the virtues you enjoy.Thus it is, therefore, that Scripture recounts that some of the patriarchs had many wives at the same time, that others took other wives when previous wives had died. The purpose of this is to indicate figuratively that some can exercise many virtues at the same time; others cannot begin those that follow before they have brought the former virtues to perfection. Accordingly Solomon is reported to have had many wives at the same time, to whom the Lord had said, “There was no wise man like you before you and there will not be after you.” Because therefore the Lord had given him an abundance of prudence, “like the sand of the sea,” that he might judge his people “in wisdom,” for this reason he could exercise many virtues at the same time.

[AD 253] Origen of Alexandria on Genesis 25:1
However, beyond this which we are taught from the law of God, if we also are in touch with some of these instructions that appear to be on the outside in the world—for example, as the knowledge of literature or the theory of grammar, as geometry or mathematics or even the discipline of dialectic—and we bring over to our purposes all these things which have been sought from without and we approve them in the declaration of our law, then we will appear to have taken in marriage either foreign wives or even “concubines.” And if, from marriages of this kind, by disputing, by discussing, by refuting those who contradict, we shall be able to convert some to the faith, and if, overcoming them with their own reasonings and skills, we shall persuade them to receive the true philosophy of Christ and the true piety of God, then we shall appear to have begotten sons from dialectic or rhetoric as if from some foreign wife or concubine.

[AD 373] Ephrem the Syrian on Genesis 25:1
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 175 years old and was buried next to Sarah, his wife.

[AD 420] Jerome on Genesis 25:1
(Chapter 25, Verse 1) And Abraham added and took another wife, and her name was Keturah. And she bore him Zimran, Jokshan, Medan, Midian, Ishbak, and Shuah. And Jokshan begot Sheba and Dedan. And the sons of Dedan were Asshurim, Letushim, and Leummim. And the sons of Midian were Ephah, Epher, Hanoch, Abida, and Eldaah. All these were the sons of Keturah. And Abraham gave everything that he had to Isaac. But to the sons of the concubines whom Abraham had, Abraham gave gifts, and he sent them away from Isaac his son, while he was still alive, to the east, to the land of the east. Cetura is interpreted in the Hebrew language as 'joined' or 'bound'. For this reason, the Hebrews suspect that, with a changed name, she is the same as Hagar, who, after the death of Sara, went from being a concubine to a wife. And it seems that the advanced age of Abraham is now being excused, so that he is not accused of indulging in new marriages after the death of his old wife. Leaving aside what is uncertain, we say this: the sons of Abraham born from Cetura, according to Hebrew historians, occupied Troglodytis and Arabia, which is now called Idumea, up to the borders of the Red Sea. But it is said that one of the descendants of Abraham, who was called Apher, led an army against Libya, and there, after defeating the enemies, settled. And his descendants from the name of their ancestor were called Africa. This fact is testified by Alexander, who is called Polyhistor, and Cleodemus, nicknamed Malchus, who retell the barbarian history in Greek. And as for what he says, 'And the sons of Dedan were Asshurim, and Letushim, and Leummim': they think that Asshurim refers to merchants, Letushim to those who work with copper and iron, and Leummim to chiefs, meaning leaders of many tribes and peoples. Others claim that they are called Asurim, that they are Syrians, and that the regions of India have been occupied by most of the sons of Abraham from Keturah.

[AD 430] Augustine of Hippo on Genesis 25:1
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.