Augustine of Hippo
The offspring of concubines also possess certain gifts. However, neither heretics nor those who are fleshly in nature will enter the promised kingdom, as there is only one heir, Isaac. It is not the children of the flesh who are regarded as the children of God, but rather the children of the promise (Romans 9:8). It is stated, "In Isaac shall your seed be called" (Genesis 21:12). I see no reason why Hittura, who married after the passing of her wife, is referred to as a concubine, except for this specific mystery. Even those who refuse to acknowledge this symbolism should refrain from criticizing Abraham. Might it be that this serves as a caution to future heretics, who oppose second marriages, by illustrating through the father of nations that remarrying after the death of a spouse is not sinful?