God promised Abraham that his seed would create a new nation. But there was a problem—Abraham had no children because his wife, Sarah, was barren. At age 75, he left Ur and journeyed to Canaan with his 65-year-old wife, Sarah. It is interesting that God never revealed the visible sign or token of His covenant to Abraham until Abraham was 99 years old.
The secret sign of the covenant was circumcision, which involved removing the foreskin from the male child. The practice of circumcision was known in Abraham's day among the Egyptians and other Semitic groups. However, the Almighty's sign was different for two reasons. The ancient Semites would often cut a mark in the male foreskin, but God said the foreskin must be completely removed. Secondly, performing circumcision must be done on the eighth day after a son's birth:
"This is My covenant which you shall keep, between Me and you and your descendants after you: Every male child among you shall be circumcised; and you shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between Me and you. He who is eight days old among you shall be circumcised, every male child in your generations, he who is born in your house or bought with money from any foreigner who is not your descendant" (Gen. 17:10-12).
Why did God wait for 24 years until revealing the sign of the covenant to Abraham? I believe it was because of Ishmael.
After Abraham failed for 11 years to have a child through Sarah, Sarah finally suggested that Abraham impregnate Hagar, her Egyptian handmaid. Hagar conceived and bore Abraham a son named Ishmael, meaning, "God has listened." Abraham was 68 when Ishmael was born (Gen. 16:16). At age 99, God revealed that circumcision was the sign of the covenant (Gen. 17:10-12).
God waited until Ishmael was 13 years of age before the revelation of circumcision was given to Abraham. In Judaism, the age 13 is considered the coming of age for boys. Jews conduct a bar mitzvah for young boys turning 13, a special ceremony indicating the child is now responsible for his own actions. God waited until Ishmael was 13, indicating that Ishmael was now responsible for his own spiritual walk with God.
Consider this. If at age 75 Abraham had known that circumcision was the sign of the covenant, then 11 years later the son of the bondwoman would have been "marked" for God's covenant promise, thus disrupting God's plan of a Hebrew nation through Abraham and Sarah. Thus God waited until Ishmael could take personal responsibility for his own moral and spiritual destiny (age 13).
Then at age 99, Abraham received the seal of the covenant. One year later Isaac was born (Gen. 21:3-5). The mark of circumcision is important to the Jewish people, since those who are unborn are considered the "seed" of future generations. The term "seed" is often used in the Torah to identify unborn children, since conception occurs after the seed (sperm) of the man passes through the covenant "mark" (on the foreskin).
Circumcision on the eighth day is interesting for several reasons. First, Jewish tradition believes that the first seven days of an infant's life represent the finished creation of the physical world in seven days. The number eight (representing new beginnings) transcends the physical world and initiates the child into the Abrahamic covenant. Also, a newborn was to experience one Sabbath before being circumcised.
Muslims choose the seventh day based upon statements in the Islamic traditions called the hadith. In America, circumcision is usually performed at a parent's request within two days of the child's birth, but Torah-observant Jews keep the commandment to circumcise on the eighth day.
Medical research has discovered two unique blessings linked to circumcision. The British Journal of Cancer reported that certain cancers of the cervix appear lower in Jewish women in Israel than among other female ethnic groups. Some have suggested that circumcision assists in actually preventing cervical cancer in Jewish women.
The second feature is a blood-clotting factor, vitamin K that contains prothrombin. It appears (based on data) that on the eighth day after birth, an infant baby has more available prothrombin than on any other day of his life, making the eighth day the best time for circumcision. Thus, God knew the physical and medical significance for eighth-day circumcision.
After Isaac was weaned, Ishmael was removed from the household of Abraham, which gave full covenant rights to Abraham and Sarah's promised son (Gen. 21:10-14).