Augustine of Hippo
What implications arise from the fact that Abraham took Keturah as his wife following Sarah's death? We should not hastily conclude that this action reflects a lack of self-control, particularly given his advanced age and profound faith. Was it truly necessary for him to father more children when Isaac, whose faith had been thoroughly tested, was already destined to have descendants as numerous as the stars in the sky and the grains of sand on the shore (see Genesis 22:17)? The apostle indicates that Hagar and Ishmael represent the fleshly individuals of the Old Covenant (see Galatians 4:24, 29); therefore, could it not also be the case that Keturah and her offspring symbolize those fleshly individuals who mistakenly believe they are part of the New Covenant?
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Both Hagar and Keturah are referred to as wives and concubines of Abraham, while Sarah is solely recognized as his wife. In the instance of Hagar, the scripture states: And Sarah, Abram's wife, took her handmaid, Hagar the Egyptian, after ten years of Abram's sojourn in the land of Canaan, and gave her to Abram her husband to be his wife (Genesis 16:3). In relation to Keturah, who was wed after Sarah passed away, we read, Abraham took another wife, named Keturah. In both cases, they are referred to as wives, yet later on, they are also recognized as concubines.