Adam Clarke on Deuteronomy 10:16

Circumcise - the foreskin of your heart - A plain proof from God himself that this precept pointed out spiritual things, and that it was not the cutting away a part of the flesh that was the object of the Divine commandment, but the purification of the soul, without which all forms and ceremonies are of no avail. Loving God with all the heart, soul, mind, and strength, the heart being circumcised to enable them to do it, was, from the beginning, the end, design, and fulfillment of the whole law.