“For everything there is a season, and a time for every purpose beneath the heavens” (Ecclesiastes 3:1) – there was a set time for Abraham when circumcision was given to him, as it is stated: “On that [hazeh] very day,2The word hazeh indicates that it was a preordained time. Abraham was circumcised, and Ishmael his son” (Genesis 17:26). There was a set time for his descendants, who underwent [mass] circumcision twice, once in Egypt3Before offering the Passover sacrifice, which could not be eaten by the uncircumcised (Exodus 12:48). and once in the wilderness,4In the time of Joshua (Joshua 5:3). as it is stated: “All the people who went out were circumcised [but all the people born in the wilderness, on the way as they came out of Egypt, had not been circumcised]…” (Joshua 5:5).
He [Abraham] could have been circumcised at age forty-eight when he came to know his Creator.5See Bereshit Rabba 30:8. Why did God wait until Abraham was ninety-nine years old to command him to circumcise himself? However, this was not done so as not to close the door in the face of proselytes.6It is to show them that even at an advanced age it is possible to be circumcised and convert. And if you ask: He could have been circumcised at age eighty-five,7Before Yishmael was conceived. [or]8This word is added by Rashash. when He spoke with him [at the covenant] between the pieces,9At age seventy (Seder Olam chapter 1). this was not done so that Isaac10Isaac alone, and not Yishmael. would emerge from a sacred drop.11So that Isaac’s conception, but not Yishmael’s, would be after the circumcision, in order to endow Isaac with special sanctity.
He could have been circumcised at age eighty-six, after the time Ishmael was born. Reish Lakish said: [God said:] ‘I am producing cinnamon in the world.’12The righteous are compared to spices. Just as with cinnamon, as long as you fertilize it and hoe it,13Even when it is very old and appears to be beyond hope for further growth. it yields fruit; so it was [with Abraham,] when his blood had become sluggish, his evil inclination depleted, his desire dissipated, when his blood had thickened.14When he was least likely to father a child.
He said: ‘If circumcision is so dear [to God], why was it not given to Adam the first man?’15This is a truncated version of the discussion related in Bereshit Rabba 11:6.