What does Genesis 21:4 mean?
On the very same day when God had changed Abram's name to Abraham, and Sarai's name to Sarah, He told Abraham that Sarah would have a son within a year and they would call him Isaac. Abraham had laughed (Genesis 17:15–17). On that day, God had also commanded Abraham to be circumcised, along with every male in his household, and to circumcise all future male children born to his household at eight days old (Genesis 17:10–13).The act of circumcision involves removing the foreskin from the penis, cutting in a circle around the organ. This ritual is deeply symbolic, echoing God's sovereignty over fertility and children, the uniqueness of His people, and the permanence of His influence. God would continue the requirement for circumcision in the laws given to Israel through Moses (Leviticus 12:3).
Now Abraham obeys that command with this promised son Isaac, circumcising him on the eighth day of his life.