The deity appears to Abram at the age of 90 and cuts another deal with him: Abram's job is to be blameless and walk with the Lord. In return, the deity will make Abram's lineage great.
Then—big reveal—the Lord changes Abram's name to Abraham. Aha. We've been waiting for that.
Abraham will spawn several nations and kings.
But for Abram's part, every dude needs to be circumcised. Circumcision is the mark of the deal between the Lord and the descendants of Abraham, and it must be done on the eighth day after the baby is born.
Every male must be snipped.
He who is not snipped "shall be cut off from his people," for he's broken the deal with the deity (17:14 NRSV).
While he's at it, God changes Sarai's name to Sarah. She will have a son, and her descendants will form nations and become kings.
Abraham's rolling on the ground laughing his pants off. She's 90. How on earth will she have a son? (We're with him.)
God insists that Sarah will bear a son and declares the boy's name will be Isaac. Sure, God. Sure.