Also the inward circumcision is necessary, the circumcision of the heart. It was commanded to the people, that they should circumcise their hearts. Circumcise therefore the foreskin of your heart, and be no more stiffnecked. The people were circumcised, but were also stiffnecked. Outward circumcision and being stiffnecked easily go together.
Inward circumcision is necessary, to put off that stiffneckedness.
We are not able to circumcise our hearts. Only God can. Also the other prophets speak of the circumcision of the heart. But at the same moment, their evil was raised to such a height, that the LORD threatened that His fury would come forth like fire, when they not amended their lives. The LORD will circumcise your hearts.