And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed. He that is eight days old - Because previously to this they were considered unclean, Leviticus 12:2, Leviticus 12:3, and circumcision was ever understood as a consecration of the person to God. Neither calf, lamb, nor kid, was offered to God till it was eight days old for the same reason, Leviticus 22:27.
Related Commentaries of Genesis 17:12
Genesis 17:12
12 And he that is eightd days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed.