The Apostle Paul frequently uses the Israelite practice of circumcision as metaphor when teaching about Christ and his gospel. For instance
28 For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh:
29 But he is a Jew, which is one inwardly; and is that of the heart, in the , and not in the letter; whose praise is not of men, but of God. (Romans 2)
I have assumed that the idea of circumcision of heart was Paul's invention, but it turns out I could not be more wrong. The idea of circumcision of heart is in fact more commonly conveyed in the Old Testament than in the New Testament!
therefore the foreskin of your , and be no more . (Deuteronomy 10:16)
And the Lord thy God will thine heart, and the heart of thy seed, to the Lord thy God with all thine heart, and with all thy soul, that thou mayest live. (Deuteronomy 30:6)
yourselves to the Lord, and take away the foreskins of your heart, ye men of Judah and inhabitants of Jerusalem: lest my fury come forth like fire, and burn that none can quench it, because of the evil of your doings. (Jeremiah 4:4)
25 Behold, the days come, saith the Lord, that I will punish all them which are with the uncircumcised;
26 Egypt, and Judah, and Edom, and the children of Ammon, and Moab, and all that are in the corners, that dwell in the wilderness: for all these nations are uncircumcised, and all the house of Israel are uncircumcised in the heart. (Jeremiah 9)
In that ye have brought into my sanctuary , uncircumcised in heart, and uncircumcised in flesh, to be in my sanctuary, to it, even my house, when ye offer my , the and the , and they have broken my because of all your abominations. (Ezekiel 44:7)
And that I also have walked contrary unto them, and have brought them into the land of their enemies; if then their uncircumcised hearts be , and they then accept of the of their iniquity (Leviticus 26:41)
Paul is really just repeating Old Testament teachings: The circumcision of the foreskin is merely an outward expression of an inward covenant commitment. As the Lord explains to Jeremiah in Jeremiah 9 quoted above, the circumcised will be punished with the uncircumcised because they are uncircumcised in heart and therefore their circumcision is hollow and useless. The heart in Hebrew may refer to the literal organ, but is most commonly understood as our inner control center, where our words, thoughts and true desires originate. (See this post).
These ideas and phrases are also found in the Book of Mormon
Wo unto the of heart, for a knowledge of their iniquities shall smite them at the last day. (2 Nephi 9:33)
But Nephi said unto them: O ye , ye uncircumcised of heart, ye blind, and ye people, do ye know how long the Lord your God will suffer you that ye shall go on in this your way of sin? (Helaman 9:21)
There is a twist in Jeremiah 6:10, talking about uncircumcised ears
To whom shall I speak, and give , that they may hear? behold, their ear is , and they cannot hearken: behold, the word of the Lord is unto them a reproach; they have no delight in it.
And in Exodus 6:12, when Moses laments his speaking disabilities, the Hebrew literally reads "uncircumcised lips".
Circumcision is one of many specific and detailed instructions that the Israelites were commanded. It is clear that such instructions were not expressions of a petty God who just wanted to be obeyed. They carried deeper meaning, had a spiritual dimension, and were given for the benefit of the receiver. Circumcision represents a covenant dedication of our inner being/hearts, including our ears and mouth and all that we are.