peritomē

Circumcision was the physical sign of the covenant God made with Abraham, marking every male on the eighth day (Genesis 17). The New Testament uses the word first for that literal rite and for the permanent mark it leaves on the body.

From there the word stretches. Paul repeatedly weighs the bodily mark against the uncircumcised state and concludes that the outward cutting counts for nothing on its own: what matters is a life that keeps faith with God. So the same word that names a real surgical rite can be emptied of value in an argument about what actually pleases God.

The word then turns inward. Building on the Hebrew Scriptures, which already called for a heart set right toward God, the New Testament speaks of a circumcision not made by human hands but worked by God's Spirit, one that cuts away the old self rather than a piece of flesh.

Because circumcision marked out the covenant people, the single word came to stand for the people themselves. "The circumcision" simply means the Jews, in the same way "the uncircumcision" means the other nations. The sentence is then about two groups of people, not about surgery.

Paul uses the word this way constantly. The gospel "to the circumcised" is the mission to the Jewish people, and Christ became "a servant to the circumcised" for the sake of God's promises to them. The word names a whole people by their defining sign.

In a few places the phrase "those of the circumcision" narrows to a faction: the group in the early church that pressed Gentile believers to be circumcised as well. Context, not the word itself, tells you whether it means the Jewish people in general or that particular party.