Genesis 17:14
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. 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.
The command was given when Abraham had lived nearly twenty-five years in Canaan, when God told him to walk before Him and be perfect and renewed the covenant that he would be a father of many nations, changing Abram to Abraham to signify father of a great multitude and Sarai to Sarah to signify princess, because she would be a mother of nations with kings of people coming from her. Circumcision was then required of Abraham and his descendants as a seal of the righteousness of the faith which Abraham already had while still uncircumcised.
Circumcision was the token or sign of the covenant in the sense of what confirms, ratifies, and makes the covenant stable and sure, proved by the fact that whoever lacked the token was cut off. It was an institution and a rite, a memorial or reminder. It was a sign in the flesh, whereas the Sabbath is a sign in the mind.
The sign marked Abraham's descendants as devoted to God's service, separated from idolaters, and accepted by God as His peculiar treasure, and it pledged them on their part to fulfill the conditions of the covenant by remaining separate from other nations and faithful to God. Keeping that pledge included not contracting marriages with the heathen, because by doing so they would lose reverence for God and His holy law and be tempted to engage in the sinful practices of other nations and be seduced into idolatry.
An uncircumcised male child breaks the covenant and is cut off from his people, so that circumcision was the condition by which a child could be entitled to the blessings of God's covenant with Israel, and neglect of it brought the Lord's displeasure and danger of immediate destruction even upon the family of God's chosen leader. Mixing with surrounding nations exposed the descendants to idolatry and caused them to lose their distinctive holy character, fall into idolatry, and go down into Egypt, bringing hardship, yet God did not forsake them because of His covenant with Abraham. Had they kept the covenant they would not have fallen into idolatry or gone down into Egypt. Had they kept it there would have been no need for God to proclaim the law at Sinai and add further directions to guard it.
Moses, yielding to his wife's persuasion, failed to circumcise his youngest son, which lessened the force of the divine precepts upon the people and left Moses without security, because while living in neglect of a known duty he could not be shielded by angels, whose protection alone could preserve his life in great peril, until the rite was performed. The same law applies in the time of trouble just before the coming of Christ, when the righteous will be preserved through the ministration of heavenly angels but there will be no security for a transgressor of God's law, since angels cannot protect those who are disregarding even one divine precept.