Circumcision was applied to all Abraham's children, regardless of whether they loved and believed in God or not. Isaac did, Ishmael didn't. Circumcision was prescribed to all ethnic Jews until the Messaih should come. It was prescribed to show that the Saviour would be born of a women and to encourage people to look to Abraham, who was a man of faith.
Now that Chrsit has come, literal circumcision has ceased.
As the Bible says in the New Testament: 'For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.
That covenant between God and Abraham ceased, which is why we opt out.