What Does Genesis 21:7 Mean?

And she added, "Who would have said to Abraham that Sarah would nurse children? Yet I have borne him a son in his old age." Sarah's rhetorical question, "Who would have said?", is the expression of fulfilled impossibility. The question is not ignorance of the answer; the narrative has shown exactly who said it, and exactly as He said it, it has happened. The rhetoric of impossibility frames the fulfilled promise: what seemed so improbable that no one would predict it has occurred precisely as the divine promise specified.

The specific image of nursing, "Sarah would nurse children", brings the fulfillment to its most bodily level. The woman whose womb was closed now nurses the child whose birth was declared impossible. The same body that was the obstacle to the covenant's fulfillment has become the means of the covenant heir's sustained life. The theological event has a physical, embodied, immediately experienced reality in Sarah's nursing of Isaac.

The nursing of Isaac by Sarah is the bodily evidence that the covenant genealogical line runs through Sarah's body, not through a substitute. The surrogacy of chapter 16 produced Ishmael through Hagar but did not produce the covenant heir. The nursing mother is the biological mother; the distinction will become the ground of conflict in the next verses when the weaning feast occasions the confrontation over inheritance.