Circumcision Is of the Heart
In ancient Israel no one could come into the temple and worship God unless he was a national Jew and circumcised in the flesh. This fleshly circumcision was required because the first covenant was a fleshly covenant with physical promises. That is why a New Covenant was necessary; physical human beings could not keep a Spiritual law or develop Spiritual character without an added spiritual dimension to his physical mind.
That added spiritual dimension came on the day of Pentecost with the outpouring of God's Holy Spirit. These were Jewish converts receiving God's Spirit on that day of Pentecost. But God knew that the New Covenant was to be global in nature and accept Gentiles as well as Jews. God made this abundantly clear when He sent Peter and witnesses to the home of a Roman centurion named Cornelius (Acts 10:34-38). As God poured out His Spirit upon that household, the circumcision (Jews) present were absolutely astonished that the Gentiles were now accepted to approach God just as they were. For the first time in all history, the Gentile converts could approach God and be accepted just as the Jews BECAUSE OF CIRCUMCISION OF THE HEART. Read Romans 2:25-29, especially v. 28-29. "For he is not a Jew, which is one outwardly (born into the Jewish race); neither is that circumcision, which is outward in the flesh: But he is a Jew, which is one INWARDLY (heart and mind); and circumcision is that of the heart IN THE SPIRIT, and not in the letter."
Why Circumcise the Heart?
There is an arch enemy of God and the human race, known as Satan the Devil (Rev. 12:9). Satan and his angels were once followers of God and creations of God. Satan became envious and jealous of God; he wanted to BE God (Ezek. 28:12-17; Isa. 14:12-14). Apparently Lucifer, whose name was changed to Satan, convinced about 1/3 of all angels to join him in rebellion against God and tried to overthrow God as ruler of the universe. With Satan and his angels defeated and knowing their fate, they developed such hatred, enmity and hostility toward God that all evils on the earth are results of their efforts to keep mankind from becoming what they could not - SONS OF GOD.
Ephesians 2:1-3 begins to unfold why we need circumcision of the heart. "And you hath he quickened, who were dead in trespasses and sins: Wherein in time past ye walked according to the COURSE OF THIS WORLD (environmental influences for evil), according to the PRINCE OF THE POWER OF THE AIR, THE SPIRIT (Satan) that now worketh in the children of disobedience: Among whom also we all had our conduct in times past in the LUSTS OF THE FLESH, fulfilling the DESIRES OF THE FLESH and of the MIND; and were by NATURE (because of Satan and his angels and the environmental influences around us) the children of wrath, even as others." But notice, it is Satan who is the prince of the power of the air that is filling this earth with his hatred, hostility, enmity against anything and everything that is Godly; we as human beings have developed what is known as human nature; but it is not human nature, it is Satan's nature of resenting God and everything that is good and wholesome and righteous. As a result of Satan's nature being built into us from birth through his deception and the environment he has created through his deception, we constantly are fulfilling the lusts of the flesh and the mind, which are CONTRARY TO GOD'S NATURE.
But notice what circumcision of the heart must accomplish or we cannot attain to the righteousness of the New Covenant. It is impossible to be a New Covenant Christian unless we are circumcised in heart. Here is why, Eph. 2:11-15, "Wherefore remember, that ye being in time past Gentiles in the flesh, who are called Uncircumcision by that which is called Circumcision (Jews) in the flesh made by hands; That at that time ye were WITHOUT Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise, having NO HOPE, and WITHOUT God in the world: But NOW (under the New Covenant) in Christ Jesus ye who sometimes were far off are made nigh by the blood of Christ. For he is our peace, who hath made both (Jew and Gentile) one, and hath broken down the middle wall of partition: HAVING ABOLISHED IN HIS FLESH THE ENMITY (hostility, hatred toward God), regulations of authoritative prescriptions contained in laws (last phrase from actual Greek meaning)."
There it is. God must circumcise the heart to cut away the enmity, hostility and hatred toward God and His Spiritual laws contained in His word; that Satan has put into us from birth through his deception, sublity and the environment; which he has molded to destroy us as potential sons of God. Without cutting away from our hearts and minds the hatred, enmity and hostility of Satan's nature developed in us toward God, we could never put on God's divine nature and be born as sons and daughters of God.