What does Genesis 17:12 mean?
God has just revealed to Abraham that He will require that he and every male member of his household be circumcised. In this way, God's people, born through Abraham, will be set apart for Him. This will be a sign of the covenant between God and Israel, starting right now. Circumcision involves the removal of the foreskin from the penis. This symbolizes the imperfection of our "natural" state, the eternal promise of God, and the influence of God in the very existence of future generations.Verse 12 gets more specific about how God expects Abraham and his family to apply this practice. The ritual of circumcision is to be performed on every male baby at eight days old. This applies to absolutely everyone who is part of the household; even male slaves, not related to Abraham by blood or marriage, must be circumcised. This requirement from God is given to Abraham centuries before the Law of Moses. Covenant circumcision is one of the foundational aspects of God's relationship with Israel.
Later verses will also specify that those who refuse this sign are to be "cut off" from the people—they cannot claim to be part of the chosen nation of God if they refuse to take on this covenant sign (Genesis 17:14). This applies to the nation of Israel, not to all Christian believers, as the New Testament will make clear.