Paul is speaking of those who belong to Christ as if they were inside Christ's body. This can also be made active. Alternate translation: "When you joined the church in baptism, God circumcised you" (See: [[rc://en/ta/man/jit/figs-metaphor]] and [[rc://en/ta/man/jit/figs-activepassive]])
With this metaphor, Paul says that God has made Christian believers acceptable to himself in a way that reminded him of circumcision, the ceremony through which Hebrew male babies were added to the community of Israel. (See: [[rc://en/ta/man/jit/figs-metaphor]])