Circumcision of the Heart is used by Paul in instructing baby Christians about the necessary doctrines.

Romans 2:17. But if you would call yourself Jewish and would cause yourself to rest upon Torah (Teaching) and to take pride in God 18. then you know His will and since you are instructed in the Torah (Teaching), you are testing the things that differ (holy and profane) 19. and you have persuaded yourself to be a guide of the blind, a light for those in darkness, 20. an instructor of the foolish, a teacher of infants, since you have the essence of the knowledge and of the truth in the Torah (Teaching): 21. then, why do you, the one who teaches another, not teach yourself? The one who preaches not to steal, are you stealing? 22. The one who says not to commit adultery, do you commit adultery? The one who detests idols, do you rob temples? 23. You who boast in Torah (Teaching), you are dishonoring God by violating the Torah (Teaching): 24. for just as it has been written, “The name of God is being blasphemed among the heathens because of you.” 25. For what does it profit if on the one hand you would practice Torah (Teaching) in circumcision: if on the other hand you would be breaking Torah (Teaching), your circumcision has become uncircumcision. 26. Therefore if the uncircumcised one would keep the things established in Torah (Teaching), will not his uncircumcision be credited to his account as circumcision? 27. And the physically uncircumcised, who are keeping the Torah (Teaching) will condemn you, although you have Scripture and circumcision, you are a violator of Torah (Teaching). 28. For the Jewish person is not the one to be plainly recognized (by what he wears, his prayer shawl or kippah) and neither is circumcision just openly in the flesh, 29. but one is Jewish inwardly, and circumcised in his heart in spirit not letter, whose praise is not from people but from God.