Gen. 17:19 "And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him. 20 And as for Ishmael, I have heard thee: Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation. 21 But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year. 22 And he left off talking with him, and God went up from Abraham."

This passage speaks of Abraham's two sons.  Ishmael was a child of the flesh.  In the manner in which Ishmael was born and the causes of his birth are identical to every one who has been born on earth with the exception of Jesus who was born of a virgin.  Ishmael was born of the flesh.  There is a sense in which Isaac was a child of the flesh as well.  Yet there is something different about Isaac's birth.  Isaac's birth was by promise from God. 

There are similarities between Isaac's birth and the spiritual birth of God's elect.  We read in Gal. 4:28: "Now we, brethren, as Isaac was, are the children of promise."  We notice the following about Isaac's birth and compare it to the spiritual birth of God's elect:

                        1.  Isaac's birth was by promise from God.  Likewise, according to the covenant of redemption God knew the elect before the foundation of the world: Eph. 1:4 "According as he has chosen us in him before the foundation of the world…" 

                        2.  Isaac's birth was contrary to nature.  He was conceived of parents whose reproductive ability was dead.  As the elect children of God we are born spiritually when we were spiritually dead: Eph. 2:1 "You hath he quickened who were dead in trespasses and sins."  A dead person has no ability to bring about life.  A person who is reproductively dead cannot bring about a natural child.  Likewise, a person who is spiritually dead cannot bring about a spiritual child.  Yet God brought about the birth of Isaac when Abraham and Sarah had no ability to bring about his natural birth.  God restored them to the time of life and they had a child.  Likewise, God brought about our spiritual birth when we had no ability to bring it about.

                        3.  Isaac's birth was at God's appointed time: "But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year."  Only God can appoint the birth of a child exactly one year later.  Prospective parents cannot guarantee that they will have a child exactly one year after they predict they will have a child.  Likewise, our spiritual birth is at God's appointed time: John 3:8 "The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit."  No man can appoint the time of his spiritual birth.  His spiritual birth occurs when God appoints that it will occur. 

                        4.  Isaac's birth was by covenant arrangement by God.  Likewise the spirit birth of the elect child of God is by covenant arrangement by God in the covenant of redemption: Rom. 8:29, 30 "For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren. 30  Moreover whom he did predestinate, them he also called: and whom he called, them he also justified: and whom he justified, them he also glorified."

Ishmael was blessed of God as God told Abraham: "Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation."  These things came to pass just like God said it would. 

God, however, had something better for Isaac: "But my covenant will I establish with Isaac."  The covenant promises made to Abraham were to be confirmed in Isaac, the child of promise. 

Gen. 17: 23-27 – Abraham obeys the commandment of God

Gen. 17:23 "And Abraham took Ishmael his son, and all that were born in his house, and all that were bought with his money, every male among the men of Abraham's house; and circumcised the flesh of their foreskin in the selfsame day, as God had said unto him. 24 And Abraham was ninety years old and nine, when he was circumcised in the flesh of his foreskin. 25 And Ishmael his son was thirteen years old, when he was circumcised in the flesh of his foreskin. 26 In the selfsame day was Abraham circumcised, and Ishmael his son. 27 And all the men of his house, born in the house, and bought with money of the stranger, were circumcised with him."

While Abraham initially laughed at the promise of God that he would have a son when he was 100 years old and Sarah was 90 years old, yet Abraham showed his faith in God and his faith that God would bring to pass what he promised as he obeyed the commandment of God to circumcise all the male members of his household, including himself.  This shows that Abraham believed God would bring to pass the covenant promise that God had made to him, including the promise of a child by Sarah when they had been before both reproductively dead.  He was persuaded that God would bring to pass what God had promised: Rom. 4:19-22 "And being not weak in faith, he considered not his own body now dead, when he was about an hundred years old, neither yet the deadness of Sarah's womb: 20 He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God; 21 And being fully persuaded that, what he had promised, he was able also to perform."