God told Abraham that he must be circumcisied as well as all the males in his family and with his family.  It was a sign of the covenant God was making with him.  God certainly could have come up with another way of marking his people. 

Instead he chose one that was private and was a reminder to the man that he belonged to God and that his strength, virility, and ability to bear fruit rested in the blessing of God Almighty.

Abraham obeyed immediately and God quickly fulfilled his promise of a baby.  Before circumcision Abraham only produced a baby by human planning.  After circumcision Abraham experienced the birth of a baby that only God could have given.

The New Testament teaches of a circumcision of the heart in Rom. 4:9-12 and Gal. 2:15-21.  God calls His people to "put off" or "cut off" something from their lives so they can bear much fruit. 

Lord, show me what needs to be "cut off" in my life so that I can bear your fruit.