Romans 4:11
And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also:
Circumcision came to Abraham after nearly twenty-five years in Canaan, at the time his name was changed from Abram to Abraham and Sarai's to Sarah, as a seal of the righteousness of the faith he already possessed while still uncircumcised. It was to be kept by the patriarch and his descendants as a token that they were devoted to God's service and thus separated from idolaters, and that God accepted them as His peculiar treasure. By this rite they were pledged to fulfill on their part the conditions of the covenant made with Abraham.
Sign, seal, mark and token are the same thing in Scripture, since circumcision is called both a sign and a seal. A seal is that which confirms, ratifies, makes stable, authenticates, gives assurance, secures and makes reliable. Circumcision was the token or sign of the covenant with Abraham, a token, institution, rite and memorial in the flesh that ratified the covenant and made it sure, so that those without it were cut off, and it served as a memorial and reminder. It was to be observed as a token of devotion to God's service and separation from idolaters, of acceptance as His peculiar treasure, and as a pledge to fulfill the conditions of the covenant on their part. That pledge included not contracting marriages with the heathen, for by doing so they would lose reverence for God and His holy law and would be tempted to engage in sinful practices and be seduced into idolatry.
The Greek word rendered sign is the same word the Septuagint uses for the mark in Ezekiel 9:4. Circumcision was an institution and rite serving as a sign in the flesh, parallel to the Sabbath which is also an institution but serves as a sign in the mind as a memorial of creation and of God's power and Godhead.
Abraham is the father of all who believe, called the Father of the faithful and the friend of God as evidence of approval of his faith. His life was appointed and recorded in sacred history to stand as an example of faith to his believing children and succeeding generations, his deep, earnest fidelity making him a representative of faith. That fatherhood was a high honor that made him the father of the people who for centuries guarded and preserved the truth of God for the world and through whom all the nations of the earth would be blessed in the coming of the promised Messiah. He was judged worthy of that calling because it was known that he would obey His voice and keep His charge, commandments, statutes and laws, would not betray the truth for selfish purposes, would deal justly and righteously, and would command his children and household after him to keep the way of the Lord to do justice and judgment.
His appointed example included forsaking his native country, wandering as a stranger in the land of his inheritance, waiting long for the birth of the promised heir, and sending away his son Ishmael at God's command. His faith had not been perfect, for he showed distrust of God in concealing that Sarah was his wife and again in his marriage with Hagar, so that he might reach the highest standard he was subjected to another test, the closest test man was ever called to endure. He is called the father of the faithful because he carried out in heart purpose the fearful test of offering his son, as fully as if he had by his own hand taken his son's life. Through that obedience he was permitted to know something of the meaning of the great sacrifice of the Father in accepting the death of His Son to save a rebellious race. Together with chapter 5, the verse connects faith with Christ's humanity grasping divinity, so that human nature through faith can grasp the divine nature and overcome.
The verse teaches implicit confidence in God, perfect obedience to His requirements, and complete surrender to the divine will, showing that nothing possessed is too precious to give to God. It calls those who profess to be Christians to follow Abraham in yielding up their dearest treasures, denying self and obeying God's voice rather than clinging to earthly things, exemplifying faith by works as Abraham did in yielding Isaac. It lays duty on Christian parents to educate children to revere God and put His claims first. His faithfulness is linked to his responsibility to direct his household, showing that parents should guide and correct their children with love and ask God for help in shaping their dispositions. It is reflected in describing the father of a family as combining faith and works, like Abraham, the father of the faithful.