Lech Lecha is one of my favorite Torah portions. We are introduced to Abraham and immediately embark on a journey with him to the land of Canaan.
In Abraham HaShem finds a righteous individual through whose offspring he will bring the revelation of the kingdom to the whole world. At the end of the portion we learn that the specific sign that God chose for his covenant with Abraham and his descendants was that of circumcision (brit milah, ברית מילה):
This is my covenant, which you shall keep, between me and you and your offspring after you: Every male among you shall be circumcised. You shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between me and you. He who is eight days old among you shall be circumcised. Every male throughout your generations, whether born in your house or bought with your money from any foreigner who is not of your offspring, both he who is born in your house and he who is bought with your money, shall surely be circumcised. So shall my covenant be in your flesh an everlasting covenant. (Genesis 17:10-13)
All Abraham’s descendants are to be circumcised on the eighth day. This mitzvah is so important that circumcision is permitted even on Shabbat. In Judaism a special ceremony is held on the eighth day wherein the boy’s family and friends gather as the procedure is performed and special blessings are recited:
Blessed are You, LORD, our God, King of the universe, Who has sanctified us with His commandments and commanded us concerning circumcision.
Blessed are You, LORD, our God, King of the universe, Who has sanctified us with His commandments and commanded us to enter him into the Covenant of Abraham our father.
Just as he has entered into the Covenant, so may he enter into Torah, into marriage, and into good deeds. 
The blessings make a profound statement about the child entering into the special covenant that God made with the Jewish people. After the ceremony a festive meal takes place.
Obviously, this mitzvah is still in place for Jewish males today. Yeshua was circumcised on the eighth day and we even see the Apostle Paul circumcising the Jewish disciple Timothy before having him accompany him on his mission to spread the gospel. In turn, circumcision, along with the rest of the Torah, was not canceled by Messiah and all Jewish followers of Yeshua are obligated to this mitzvah. But what about Gentile believers? Do they have to be circumcised?
In Acts 15, the apostles specifically rule against Gentile believers being required to be circumcised. Furthermore, the Apostle Paul states:
Was anyone at the time of his call already circumcised? Let him not seek to remove the marks of circumcision. Was anyone at the time of his call uncircumcised? Let him not seek circumcision. (1 Corinthians 7:18)
To be clear, however, neither Paul nor the apostles are talking specifically about the surgical procedure of circumcision but rather conversion to become legally Jewish, which included the act of circumcision along with the traditional covenantal blessings quoted above. On the other hand, Gentiles believers were not required to either convert or be circumcised to enter into discipleship to Yeshua.
So the question remains: Can Gentile disciples of Yeshua voluntarily take on circumcision if they desire? We are not talking about ritual conversion but rather simply the procedure of circumcision performed on the eighth day after the child’s birth. A Gentile believer might desire to observe this commandment as a sign of solidarity with the Jewish people and as a further act of non-compulsory submission to HaShem’s Torah. It is interesting that in rabbinic tradition it is believed that several non-Jewish men were born circumcised including Adam, Seth, Noah, Shem, Job, and Balaam.  We even have evidence that some of the Gentile God-fearers in the synagogues of the Second Temple Period and beyond would undergo circumcision particularly in the second generation. 
Looking again at our Torah portion we see that all Abraham’s descendants are to be circumcised. The sages debate as to who this includes. The Rambam summarizes:
The descendants of Ishmael are excluded as implied by Genesis 21:12: “It is through Isaac, that your offspring will be called.” Esau’s descendants are also excluded, for Isaac told Jacob in Genesis 28:4: “May God grant Abraham’s blessing to you and your descendants,” implying that only he is the true offspring of Abraham who maintains his faith and his upright behavior. Thus, they alone are obligated in circumcision.
Our Sages related that the descendants of Keturah who are the offspring of Abraham that came after Isaac and Ishmael are also obligated in circumcision. Since, at present, the descendants of Ishmael have become intermingled with the descendants of Keturah, they are all obligated to be circumcised on the eighth day. However, they are not executed for failure to perform this mitzvah. (Mishneh Torah, Hilchot Melachim 10:7-8) 
So while the descendants of Ishmael and Esau are excluded, Abraham’s children from Keturah (benei Keturah, קטורה בני) are not. Today it is complicated to work this out practically, but at least one Haredi Rabbi in Israel uses this situation as justification for teaching non-Jews the procedure of brit milah:
His workshops are based on Jewish halacha, he says, and because halacha permits circumcision for “bnei Keturah,” he is allowed to teach non-Jews and perform circumcisions on their children. 
I even have Gentile friends who have had their children circumcised in the traditional manner by a mohel (traditional circumciser). This was with full disclosure that they were Gentile believers in Messiah Yeshua. The mohel was happy to perform this procedure for their child on the eighth day albeit without the traditional Jewish blessings.
In summary, it is perfectly permissible for Gentile followers of Yeshua to practice circumcision and to circumcise their boys on the eighth day if they so desire.  It should be performed with the understanding that this does not make the boy Jewish and without the traditional covenantal blessings and ceremony. While it is certainly not obligatory, the outward symbolism of a fully submitted life unto HaShem and (according to some) health benefits remain. I had both of my boys circumcised on the eighth day.
At the same time every physical and spiritual descendant of Abraham is required to have a circumcised heart. Paul describes the process of Gentiles coming to Messiah in terms of a spiritual conversion:
In him also you were circumcised with a circumcision made without hands, by putting off the body of the flesh, by the circumcision of [Messiah], having been buried with him in baptism, in which you were also raised with him through faith in the powerful working of God, who raised him from the dead. (Colossians 2:11-12)
Whatever Gentiles choose regarding circumcision is up to them, but may we all allow the Spirit of God to circumcise our hearts daily in our service to King Messiah.
- Translation from: http://www.chabad.org/library/article_cdo/aid/144464/jewish/The-Circumcision-Procedure-and-Blessings.htm.
- Avot deRabbi Natan 2; Midrash Tanchuma 5.
- Shlomo Pines, “The Iranian Name for Christians and the ‘God-Fearers,’” Proceedings of the Israel Academy of Sciences and Humanities 2 (1968): 143-152. Also according to the Talmud there were Gentiles not associated with Judaism who were also circumcised (b.Nedarim 31b).
- Translation from: http://www.chabad.org/library/article_cdo/aid/1188355/jewish/Melachim-uMilchamot-Chapter-10.htm.
- It is beyond the scope of this article to discuss the issue of whether or not a circumcision performed on a Gentile supercedes Shabbat as it would if it was performed on a Jew. In the case of a Jewish male, it is a commandment, whereas with a Gentile it is not.
Source: First Fruits of Zion