As Israel advanced into the Promised Land, the Lord wanted to ensure that their hearts were fully invested into trusting and obeying Him...unlike their forefathers. So He reinstituted the act of circumcision, which indicated that they were set apart to Him. Circumcision required personal sacrifice and pain, because that is what true investment requires. For disciples of Jesus Christ, there needs to be the same investment--what we would call "skin in the game". Romans describes this as the "circumcision of our hearts"--which means living completely surrendered to the Lord.