Inheritance and Covenant

[Sarah] said to Abraham, "Cast out this slave woman with her son, for the son of this slave woman shall not be heir with my son Isaac."
Inheritance and Covenant
What [Sarah] says about the inheritance should not be understood as though she wanted Ishmael to be excluded from the inheritance; she simply says that he should not be made an heir together with Isaac, but that Isaac should keep his portion for himself. . . She is speaking under the influence of the Holy Spirit, and she relies on the sure promise which she had heard concerning her son. Ishmael, too, had his promise; for the Lord said to Abraham (Genesis 17:20): "I have heard you praying for Ishmael, for of him shall be born twelve princes." But Sarah noted more carefully that an additional statement was made about her son with these words: "But I will establish My covenant with Isaac." . . . Abraham had not given such careful consideration to the promise. Therefore God repeats it and once more expressly states: "Through Isaac shall your descendants be named" [Genesis 21:12]. Accordingly, He does not condemn the saintly will and the just sentiment that Abraham loves his wife and son; but He merely reminds him of the promise which gave to Ishmael the hope of becoming a great nation. The covenant, however, He reserved for Isaac alone. . .. The promises were twofold. The temporal one had concerned Ishmael; the eternal and spiritual one had concerned Isaac.
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