A more literal translation from the original Hebrew is: “And you must circumcise the foreskin of your hearts and not harden your necks any longer,” (De. 10:16, Reference Bible, 1984 revision).
Under Mosaic Law, Israelites were obligated to remove the foreskin of a baby boy’s penis eight days after birth (Le. 12:2,3).
However, physical circumcision was not the key to salvation.
They needed to be sincere in their dealings toward one another and accept God’s direction.
To “circumcise” the heart means to ‘love Jehovah with all your heart and all your soul,’ (De. 30:6).
This implies getting rid of immoral or arrogant attitudes which could keep God’s spirit from tapping into our thinking and motivating good deeds (Acts 7:51).
The aforementioned passage continues:
“Jehovah your God […] executes justice for the fatherless child and the widow and loves the foreign resident, giving him food and clothing. You too must love the foreign resident […]”(De. 10:17-19).
That is why the covenant Christ made with his followers did not call for a physical mark such as circumcision (John 13:35).
Rather, Christians are obligated “to look after orphans and widows in their tribulation, and to keep oneself without spot from the world,” (Jam. 1:27).
In effect, “real” circumcision means to render sacred service with a clean heart by God’s spirit (Php. 3:3).