Synopsis of the Books of the Bible, by John Nelson Darby, [1857-62], at sacred-texts.com
The following commentary covers Chapters 20 and 21.
In chapters 20, 21 we have the question of the heir and of the path of faith in another point of view. Abraham denies his relationship with his wife, and is reproved by the world itself, which knows better than he what she should be. God, however, guards the promises in His faithfulness, and judges that which meddles with her who has to say to them. The heir of promise is born; and the heir according to the flesh, son of the bondwoman or of the law, is entirely rejected. Now Abraham reproves the powerful of the earth, before whom he had previously denied his relationship with his wife.
But these two chapters must be somewhat more developed. Like Abram's going down to Egypt, we have unbelief working in respect of the path into which he had been called by grace, shewn, as it ever is, in reference to walking in the intimacy of the relationship in which God had set him, of which woman is the expression in the types. Here Sarah is the mother of the heir of the world, the wife of Abraham, according to promise, and, for Abraham, according to the church's hope, as we have seen (though Israel were the vessel according to flesh). This position he denies. Sarah is again his sister. This was worse than before, for she is, to faith, mother of the heir of the world. Abimelech was wrong, and acted to please himself, but acted unconscious of it. Abraham before God was in the falser position of the two. God warns Abimelech, and preserves Sarah by His own power, whom Abraham's want of faith had connected with the world; and Abimelech returns her, with the cutting reproof to the church, as here typified, that she at least ought to have known her own relationship to Christ. Still, in the main, Abraham was in the place of faith and blessing; and, as God's prophet, to whom none should do harm, intercedes for the faulty Abimelech, for here all is grace. There is another point to notice here, that this was an arrangement of unbelief when first he started from his father's house (Gen 20:13), so soon was the germ of unbelief at work in the called of promise. But God maintains the divine title to the allegiance of the church at all times. But now the heir is born, the heir of promise.
The effect of this is, that not only is the difference known to faith, but the heir of the bondwoman is utterly cast out as to the inheritance. Historically he is preserved according to God's promise, a figure of legal Israel; but, as regards any portion of the inheritance, wholly cast out.
And here, further, Abraham fears no longer before the prince of this world, but reproves him. He has the world, as well as the heavenly communion, now that the heir is come; and the . world owns that God is with him in all things. Hence the well of the oath is the witness of Abraham's title in the world, and Abimelech's owning God to be with him. There, according to the oath and his title thus owned by the world, he plants a grove, takes possession of the earth, and worships, calling on the name of the everlasting God of Him who had once promised to Israel, and never abandoned His purpose, and had now accomplished on the earth what His mouth had spoken: not, indeed, so blessed a portion as the heavenly intercourse and possession of faith, but a proof of the unchangeable faithfulness of the God who had given the promises. There Abraham, in figure, now abides, where the power of the world had been. This will belong to Israel in the letter, but we, on whom the ends of the world are come, have it in a higher and better way. It was the pledge of what should be and will be; our hope is transferred to heaven where Christ is gone. But we reign there in a better way.