The command to circumcise:
Genesis 17:10, 12 This [is] my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised. And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which [is] not of thy seed. 

The purpose of circumcision: God chose Abraham and gave him the rite of circumsion which was at their time a sign that those who received it were devoted to the service of God. It was for them a pledge that they would remain separate from paganism, idolatry but rather obey the law of God. It was also a seal of the righteousness of the faith and was to be observed by him and his descendants as a token that they were devoted for God's service alone. They were also not to contract marriage with unbelievers for by doing so they would lose their reverence for God and His holy law. Therefore, if man had kept the law, there would have been no necessity for the ordinance of circumcision. In short, circumcision was introduced because of disobedience.

What is circumcision to us after Jesus Christ died on the cross?
1 Corinthians 7:19 Circumcision is nothing, and uncircumcision is nothing, but the keeping of the commandments of God. 

"There is neither Greek nor Jew, circumcision nor uncircumcision" (Colossians 3:11)

If one is not circumcise, can he still be considered as a Jew?
Romans 2:29 But he [is] a Jew, which is one inwardly; and circumcision [is that] of the heart, in the spirit, [and] not in the letter; whose praise [is] not of men, but of God.

Colossians 2:11 In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ:

What is the real essence circumcision?
Obedience: Deuteronomy  10:12-13, 16 And now, Israel, what doth the LORD thy God require of thee, but to fear the LORD thy God, to walk in all his ways, and to love him, and to serve the LORD thy God with all thy heart and with all thy soul,  To keep the commandments of the LORD, and his statutes, which I command thee this day for thy good?   Circumcise therefore the foreskin of your heart, and be no more stiffnecked.

Deuteronomy  30:6 And the LORD thy God will circumcise thine heart, and the heart of thy seed, to love the LORD thy God with all thine heart, and with all thy soul, that thou mayest live.

Jeremiah 4:4 Circumcise 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. 

What is the real essence of uncircumcision?
Disobedience: Acts  7:51 Ye stiffnecked and uncircumcised in heart and ears, ye do always resist the Holy Ghost: as your fathers [did], so [do] ye.