Albert Barnes on 1 Corinthians 7:18

Is any man called. Does any one become a Christian. 1Cor 7:26.

Being circumcised. Being a native-born Jew, or having become a Jewish proselyte, and having submitted to the initiatory rite of the Jewish religion.

Let him not become uncircumcised. This could not be literally done. But the apostle refers here to certain efforts which were made to remove the marks of circumcision which were often attempted by those who were ashamed of having been circumcised. The practice is often alluded to by Jewish writers, and is described by them. Comp. 1 Mac. i. 15. It is not decorous or proper here to show how this was done. The process is described in Cels. de Med. 7.25. See Grotius and Bloomfield.

Is any called in uncircumcision? A Gentile, or one who had not been circumcised.

Let him not be circumcised. The Jewish rites are not binding, and are not to be enjoined on those who have been converted from the Gentiles. Rom 2:27, seq.

(e) "uncircumcision" Acts 15:1, Gal 5:2