Thats a good question I dont know but thats what God wanted. Its God who establishes the covenant, not Abraham. And He wanted to do it through Isaac.
God's eternal covenant and salvation through Christ was through Isaac. But I understood that his covenant to be Israel's God, and for them to be His people, was open to all Abraham's household. Else circumcision would only make sense for Abraham's direct descendants, but not the rest of his household.
Genesis 7-8 And I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant,
to be a God unto thee, and to thy seed after thee. And I will give unto thee, and to thy seed after thee, the land wherein thou art a stranger, all the land of Canaan, for an everlasting possession;
and I will be their God.
Genesis 12-14 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 born in thy house, and he that is bought with thy money, must needs be circumcised: and my covenant shall be in your flesh for an everlasting covenant. And
the uncircumcised man child whose flesh of his foreskin is not circumcised, that soul shall be cut off from his people; he hath broken my covenant.
I gues abrahams circumcision, Ishmaels and all others were a 'token' circumcision. Wheras Isaacs circumcision was the one God wanted (at eight days old) to establish.
Aren't all physical circumcisions 'token' circumcisions, but the true circumcision is what is done in our hearts?
Why eight days, I think maybe to signify that he was perfect after 6 days creation and a days rest perhaps?
I agree. I think just as God created over 7 days, the 8th day is like a new beginning. I've also read from a biological perspective, the 8th day is the best day to circumcise due to the levels of blood clotting vitamin K being at its peak.
And also maybe to abraham wanted to be obedient, which is why he circumcised everyone in his household.
Agreed again. But God commanded that uncircumcision was breaking His covenant, so it would imply that by undergoing circumcision, Abraham's entire household were partakers in the covenant (because you can't break a covenant if you don't have one to begin with).