John Calvin Commentary 1 Corinthians 7:18

"Was any man called being circumcised? Let him not become uncircumcised. Hath any been called in uncircumcision? Let him not be circumcised." — 1 Corinthians 7:18 (ASV)

Circumcised, etc. Having mentioned the calling, he takes the opportunity, from a particular instance, to digress briefly into a general exhortation, as he is accustomed to do in many instances; and at the same time, he confirms, by different examples, what he had said concerning marriage.

The main point is this: in external matters, you must not rashly abandon the calling upon which you have once entered by the will of God. He begins with circumcisions, about which many at that time disputed. Now, he says that with God it makes no difference whether you are a Gentile or a Jew. Therefore, he exhorts everyone to be content with his condition. It must always be kept in mind that he addresses only lawful modes of life, which have God as their approver and author.