jewish leaders sometimes had a way of removing the spirit of the law and increasing the letter of the law.
circumcision was introduced as a way to cement the concept of removing the thickening from their heart. people have a way of turning their hearts away from God, a pattern of taking their own paths and getting a heart of stone towards God. God has tried to show us time and time again, even with physical reminders, that the hardening of our hearts towards him is bad. If we remove the wall setup against God, then the right things begin to happen.