Why circumcision and how is it a sign?

God commanded circumcision after Abraham laughed at His promise of a son. Abraham doubted God’s word, so God made him mark the very organ through which the promise would come. It was a covenant sign and a reminder of Abraham’s unbelief. Every time he saw it, he remembered that God’s promises do not fail. It also prefigures Christ, who was cut and bled for our salvation. The covenant mark on Abraham pointed forward to the cross.