The Jews
God had made a covenant with Abraham and his son Isaac and with his descendants after him: the Israelites. They are called Israelites because they are descendants of Israel. Israel is the name that God gave to Jacob, the son of Isaac (Genesis 32:28). The circumcision of the flesh is the sign of the covenant. Jacob had twelve sons, founders of the twelve tribes of Israel. God made the promise to Abraham that all the nations of the earth would be blessed in his seed. Now Jesus Christ is his seed; he was conceived by the Holy Spirit in Mary's womb, Joseph's wife, Joseph being a descendant of Abraham. And Jesus Christ came from God. Moses told the Israelites about Jesus. Moses told the Israelites that God would raise a prophet like him that they would have to listen to:
The LORD your God will raise up for you a Prophet like me from your midst, from your brethren. Him you shall hear (Deuteronomy 18:15, NKJV).
I will raise up for them a Prophet like you from among their brethren, and will put My words in His mouth, and He shall speak to them all that I command Him. And it shall be that whoever will not hear My words, which He speaks in My name, I will require it of him (Deuteronomy 18:18-19, NKJV).
God established a new covenant with men through Jesus Christ. In the new covenant, the circumcision is that of the heart and no longer that of the flesh (Romans 2:28-29).
Under the old covenant, the people of God are circumcised in the flesh, under the new covenant the people of God are circumcised in their heart, it is the circumcision of Christ.
In him also you were circumcised with a circumcision made without hands, by putting off the body of flesh in the circumcision of Christ (Colossians 2:11, RSV).