Genesis 17:13
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. 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.
After Abraham had lived nearly twenty-five years in Canaan, God appeared to him as the Almighty God and told him to walk before Him and be perfect, promised His covenant was with him to make him a father of many nations, changed his name from Abram to Abraham signifying father of a great multitude, and changed Sarai's name to Sarah meaning princess because she would be a mother of nations and kings would come from her. Circumcision in the flesh is the sign, token, seal, and pledge of God's covenant with Abraham and his descendants, given as a seal of the righteousness of the faith Abraham had while still uncircumcised, marking them as devoted to God's service, separated from idolaters and other nations, and accepted by God as His peculiar treasure. By receiving it they were pledged to fulfill their part of the covenant's conditions, to remain distinct and faithful to God and to be perfect.
That separation was meant to guard them from intermarriage with the heathen, idolatry, and rebellion against God, because marrying unbelievers would cause loss of reverence for God and His holy law, temptation to sinful practices of other nations, seduction into idolatry, and weakening of holy character. If Abraham's descendants had kept that covenant, they would have stayed separate from idolatry and would not have fallen into it. If they had kept it, they would not have gone down into Egypt. If they had kept it, the later proclamation of the law at Sinai would not have been necessary. If they had kept it, the additional judgments and statutes given afterward would not have been necessary.
Circumcision is a binding condition by which a child becomes entitled to the blessings of God's covenant with Israel, with no exception even for the son of a chosen leader. Yielding to persuasion to neglect it brings God's displeasure, removes security, lessens the force of divine precepts upon the people, and leaves a man without shielding by holy angels in perilous service, while prompt performance of the rite restores permission and protection to go forward. The same rule holds for every divine precept, so just before the coming of Christ the righteous will be preserved through the ministration of heavenly angels, but there will be no security for the transgressor of God's law because angels cannot protect those who disregard even one command.