17 Nevertheless, each person should live as a believer in whatever situation the Lord has assigned to them, just as God has called them. This is the rule I lay down in all the churches.
18 Was a man already circumcised when he was called? He should not become uncircumcised. Was a man uncircumcised when he was called? He should not be circumcised.
19 Circumcision is nothing and uncircumcision is nothing. Keeping God’s commands is what counts.
20 Each person should remain in the situation they were in when God called them.
in all the churches (v.17) — A general statement that what Paul has just said and is about to say isn’t only for the Corinthians.
A fine statement of this verse is found in Arthur S. Way’s translation. It reads, “Let each member go on living in the same conditions which the Lord originally allotted to him, and in which he was when he heard God’s call.” The meaning is self-evident. Conversion is not outwardly revolutionary. It is, instead, inwardly regenerative. It does not storm into our national and social life and proceed to create violent changes and adjustments. It is true that changes will occur. It leaves us totally different than what it finds us. It does not require us, however, to make an occidental of the oriental. So the principle is “as the Lord hath called everyone, so let him walk.” In other words, be what you were when Christ found you. Not, of course, in the manner of your life, but in its sphere. — Laurin, page 129.
Circumcision is nothing and uncircumcision is nothing (v.19) — This statement required a unique amount of boldness and bravery when we remember that many thousands of Jewish believers were still great zealots for the law in every detail (Acts 21:20). Circumcision in particular was considered the most basic and obligatory of all the Mosaic ordinances. Paul had formerly undoubtedly believed this also, but in the face of James and all the brethren of the law he calmly stated this imcomparable statement three times, although he knew full well what God of old had already said about this matter (1 Samuel 15:22; Ecclesiastes 12:13; Isaiah 1:11, 13, 16-17; Jeremiah 7:22; Micah 6:6-8; Hosea 6:6). It seems that even the legalistic brethren like James did not understand that the crucifixion of Christ was the real antitypical circumcision without hands, which made superfluous once and for all every rite enacted with hands (Colossians 2:10-12). — Bultema, page 59.
I think there might be reason to take a slightly different view of these verses than any of my commentaries did. Back at the beginning of my study of 1 Corinthians, I said I wanted to look for evidence that there was a mix of Jewish kingdom believers and Gentile grace believers in this church. Is it possible that these verses are confirmation of that? Later, in other letters, Paul makes it clear that there is no circumcision or uncircumcision. But here he says that they are to “abide in the same calling in which he was called.” In other words, there were those called to be of the circumcision and those called to be of the uncircumsion. In Galatians 2, Peter, James and John agree to limit their ministry to the circumcision when Paul agrees to limit his to the uncircumcision. But in the Corinthians church, there were both — as is confirmed in later passages regarding tongues. This was one of Paul’s very early letters, probably written before either Romans or Galatians, and so the mix of Judaism and grace was still evident and the book needs to be interpreted in light of that fact.