Genesis 17:9-27 NIV
 Then God said to Abraham, “As for you, you must keep my covenant, you and your descendants after you for the generations to come.  This is my covenant with you and your descendants after you, the covenant you are to keep: Every male among you shall be circumcised.  You are to undergo circumcision, and it will be the sign of the covenant between me and you.  For the generations to come every male among you who is eight days old must be circumcised, including those born in your household or bought with money from a foreigner-those who are not your offspring.  Whether born in your household or bought with your money, they must be circumcised. My covenant in your flesh is to be an everlasting covenant.  Any uncircumcised male, who has not been circumcised in the flesh, will be cut off from his people; he has broken my covenant. ”  God also said to Abraham, “As for Sarai your wife, you are no longer to call her Sarai; her name will be Sarah.  I will bless her and will surely give you a son by her. I will bless her so that she will be the mother of nations; kings of peoples will come from her.”  Abraham fell facedown; he laughed and said to himself, “Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?”  And Abraham said to God, “If only Ishmael might live under your blessing!”  Then God said, “Yes, but your wife Sarah will bear you a son, and you will call him Isaac. I will establish my covenant with him as an everlasting covenant for his descendants after him.  And as for Ishmael, I have heard you: I will surely bless him; I will make him fruitful and will greatly increase his numbers. He will be the father of twelve rulers, and I will make him into a great nation.  But my covenant I will establish with Isaac, whom Sarah will bear to you by this time next year.”  When he had finished speaking with Abraham, God went up from him.  On that very day Abraham took his son Ishmael and all those born in his household or bought with his money, every male in his household, and circumcised them, as God told him.  Abraham was ninety-nine years old when he was circumcised,  and his son Ishmael was thirteen;  Abraham and his son Ishmael were both circumcised on that very day.  And every male in Abraham’s household, including those born in his household or bought from a foreigner, was circumcised with him.
It's been 13 years since we have any record of God speaking with Abraham, 13 years since Ishmael was born as a result of Abram's lack of faith and Sarai's impatience. So when God speaks again, they're ready to listen.
However, what He tells them is difficult to take in. The promise still stands. The covenant remains. They will have a son - together - even though they're both long past it. But that's the whole point. It was never about what they could produce, but only what God would produce through them. It wasn't supposed to be about natural conception - but supernatural birth. One represents our best human efforts, the other is God's work, His enabling and empowering.
Struggling to come to terms with what he's being told, Abraham asks that God would just bless Ishmael instead. That would be easier for everyone. God responds that Ishmael will be blessed, but he won't be the fulfilment of the promise. A son called Isaac will be born in one year, with both Abraham and Sarah as parents.
Ishmael represents our best efforts to attain the promises of God through works. It produces something but never fulfils God's requirements.
Sounds a lot like religion to me.
God's way, Isaac, depends completely on Him and His power. Abraham can't produce it and Sarah definitely can't. If it's going to happen, it's going to have to be all God.
Sounds like salvation by grace alone, through faith in Jesus alone.
Abraham and Sarah still had a part to play, but the power to produce came from God alone.
God still loves to work in and through us, but especially when the results can only come through His supernatural work. If we only ever do what we can produce on our own, we will never fully experience His power. But when step out in faith beyond what is normal and natural, He adds His 'super' to make it supernatural; when we offer our best ordinary obedience, He adds His 'extra' to make our lives 'extraordinary'. That's why it's called the 'life of faith'.
When is the last time in your life when it all depended on God showing up? That you went out on a limb beyond your natural gifting or ability and threw yourself completely on His power and enabling?
The Christian life not only starts supernaturally with the new birth but it was also only supposed to continue supernaturally in the power of the Holy Spirit.