Why Only Abraham's Line Is Bound to Circumcision

Any commandment that was said to the sons of Noah and repeated at Sinai applies to both [Israel and the nations]. Now consider circumcision, which was said to the sons of Noah — "And you shall keep My covenant" — and was repeated at Sinai — (Leviticus 12:3) "and on the eighth day he shall be circumcised." One might think it was said to Israel and not to the sons of Noah! That verse "and on the day" comes to permit circumcision on the Sabbath, meaning even on the Sabbath. Or if you prefer, say: from the outset circumcision was commanded by the Merciful One only to Abraham — "And you shall keep My covenant, you and your seed after you" — yes; another person, no. But if so, let the sons of Ishmael be obligated! Scripture says (Genesis 21:12), "for in Isaac shall seed be called to you." Then let the sons of Esau be obligated! "In Isaac," but not all of Isaac. Let the sons of Keturah be obligated! "He has broken My covenant" — this comes to include the sons of Keturah.