circumcision of the heart

Chabad, Lessons in Tanya, Iggeret HaKodesh, middle of Epistle 4: 

[talking about the exile of the Shechinah as it is manifest in each soul, that is, the soul in exile to Babylon  (state of sin)] 

"Of this it is written, 'And you shall excise the foreskin of your heart'...

"In principle the spiritual service of circumcision is that of repentance. With regard to exile our Sages teach that 'If Israel repent they will be immediately redeemed'. Repentance thus leads to the redemption (on a personal scale) of the Divine spark within each individual soul and (on a cosmic scale) of the Schechinah, from their respective exiles. In spiritual terms, the act of circumcision likewise removes a veil of concealment and allows the innermost point of the heart to be revealed." 

Now, anyone who has read the New Testament is familiar with Yeshua's and Paul's teachings on repentance and circumcision of the heart: 

For a description of true holiness (circumcision of the heart) see Mattityahu/Matthew 5:17-48; 

and for the hypocrisy of the pious (maybe outwardly religious but inwardly unclean) see Mattityahu Chapter 23 

Paul to the Romans 2:28

"For the real Jew is not merely Jewish outwardly; true circumcision is not only external and physical.  On the contrary, the real Jew is one inwardly; and true circumcision is of the heart, spiritual not literal; so that his praise comes not from other people but from God." 

He goes on to explain that Abraham's righteousness was not from observance of Torah but from his faith in God. Thus, Abraham had a true circumcision of the heart, even before experiencing a physical one.