Circumcision of the heart is not a New Testament idea invented by the Apostle Paul.
“Circumcise therefore the foreskin of your heart, and be no longer stubborn." Deuteronomy 10:16
“For a person is not a Jew who is one outwardly, nor is circumcision something that is outward in the flesh, but someone is a Jew who is one inwardly, and circumcision is of the heart by the Spirit…” Romans 2:28,29
Circumcision of the heart has always been the point and the emphasis of God for his people, not physical circumcision. It was not a change from physical to spiritual between the Old and New Covenants.
What’s the point then? Obedience to the Law of God, the ethical/judicial demands of the Christian life revealed in the Word of God. No mystical dualism here. Physical circumcision was the mark of the covenant, a corporal reminder of the demands on covenant keepers.
Why is this important? Because it reflects the character of God and what he demands of us.
“Because the Lord your God is God of gods and Lord of lords, the great, mighty, and awesome God who is unbiased and takes no bribe, who justly treats the orphan and widow, and who loves resident foreigners, giving them food and clothing.” Deuteronomy 10:17,18
What is the evidence of a circumcised heart? Obedience to God's commandments, i.e. His Law.
“Now, Israel, what does the Lord your God require of you except to revere him, to obey all his commandments, to love him, to serve him with all your mind and being, and to keep the Lord’s commandments and statutes that I am giving you today for your own good?” Deuteronomy 10:12,13
The emphasis on justice, mercy and faith in the New Testament is not new. It is the basis for Jesus’ indictment of the Pharisees.
“Woe to you, experts in the law and you Pharisees, hypocrites! You give a tenth of mint, dill, and cumin, yet you neglect what is more important in the law—justice, mercy, and faithfulness! You should have done these things without neglecting the others. Blind guides! You strain out a gnat yet swallow a camel!” Matthew 23:23,24
Paul gives us an Exposition of the same in Romans 2:25-29.
“For circumcision has its value if you practice the law, but if you break the law, your circumcision has become uncircumcision. Therefore, if the uncircumcised man obeys the righteous requirements of the law, will not his uncircumcision be regarded as circumcision? And will not the physically uncircumcised man who keeps the law judge you who, despite the written code and circumcision, transgress the law? For a person is not a Jew who is one outwardly, nor is circumcision something that is outward in the flesh, but someone is a Jew who is one inwardly, and circumcision is of the heart by the Spirit and not by the written code. This person’s praise is not from people but from God.” Romans 2:25-29
Observe in context the meaning of this phrase in vs 29, “…circumcision is of the heart by the Spirit and not by the written code.” Here, the one who guards the letter of the law emphasizes physical circumcision but disobeys the law. He who is circumcised of heart keeps the law in his daily walk applying the faith to all of life.
Paul is not calling us to abandon the Law for a mystical ethic where everyone does what is right in his own eyes. In practice, such mysticism leaves us with an ethic based on convenience and sentiment, a relativism that hardly reflects the God who is the same yesterday, today and forever.
James repeats this in his call to be doers of the Word and not hearers only.
“But be sure you live out the message and do not merely listen to it and so deceive yourselves. For if someone merely listens to the message and does not live it out, he is like someone who gazes at his own face in a mirror. For he gazes at himself and then goes out and immediately forgets what sort of person he was. But the one who peers into the perfect law of liberty and fixes his attention there, and does not become a forgetful listener but one who lives it out—he will be blessed in what he does.” James 1:22-25
The law of liberty in James 1:25 is nothing less than the same Law God has revealed to man since the creation, the same Law that reflects His character, His holiness, and what he expects of us. A circumcised heart manifests in the most practical ways. Notice the parallel to the character of God in Deuteronomy 10:18
“Pure and undefiled religion before God the Father is this: to care for orphans and widows in their misfortune and to keep oneself unstained by the world.” James 1:27
“…who justly treats the orphan and widow…” Deuteronomy 10:18
These are personal responsibilities. You cannot vote them away. Voting to plunder others to help the needy is a sign of an uncircumcised heart. Like white washed graves, it is just a cover for avarice and power lust. Silence about sexual infidelity in the church and the plague of abortion is a sign of an uncircumcised heart. Silence about our unjust penal system is a sign of an uncircumcised heart. We have become like Israel in Jeremiah's day.
“Moreover, none of the people of Israel are circumcised when it comes to their hearts.” Jeremiah 9:26
How we vote, how we spend our money and time, who we hire and fire, how we treat our employees, how we work when the boss isn’t watching, how we talk about our enemies in their absence, our attitudes towards the resident foreigners in our midst, all these reflect a circumcised or uncircumcised heart. This is practical Christianity, what we mean by a faith for all of life.