Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. Neither because they are the seed of Abraham, etc. - Nor can they conclude, because they are the natural descendants of Abraham, that therefore they are all of them, without exception, the children in whom the promise is to be fulfilled.
But, in Isaac shall thy seed be called - The promise is not confined to immediate natural descent, but may be accomplished in any part of Abraham's posterity. For Abraham had several sons besides Isaac, Genesis 25:1, Genesis 25:2, particularly Ishmael, who was circumcised before Isaac was born, and in whom Abraham was desirous that the promise should be fulfilled, Genesis 17:18, and in him God might have fulfilled the promise, had he so pleased; and yet he said to Abraham, Genesis 21:12 : Not in Ishmael, but in Isaac, shall thy seed be called.