The story of Abraham teaches us that a relationship with God, wth all three Persons of the Trinity, is similar to relating to any other person: a relative, a coworker, or a friend in that relationships require active involvement of both parties. Being in a relationship with the all knowing, all powerful creator of life is clearly a subservient one. Subservient relationships, unlike peer relationships, require a particularly respectful attitude and behavior.
So far, in this tale we see God as patient and intentionally uninvolved from time to time. For example, why did Abram and Sarai have to wander so? How could God passively accept Sarai in Pharaoh's bed, and later, why didn't God warn Abram in a dream or something that long term trouble would ensue because of Sarai's impatience that lead to giving her husband over to Hagar? More importantly, why did God wait so long to give Abram a son through Sarai?
Circumstances surrounding the birth of Isaac cause us to wonder whether the birth of Jesus Christ was the first miraculous pregnancy. The birth of Isaac from a woman well past menopause with an hundred year old man, was physiologically impossible. How did Sarah get pregnant? Was the miraculous impregnation of Sarah an experiment? Was it practice for the ultimate crescendo of human history, the impregnation of the Virgin Mary?!
God's use of Abraham was more important to Himself than it was to Abraham who received little more than promises for a bright future for his lineage and some land of his own. For Abraham, that was good enough.
The lesson is that in our relationship with God, we must be extraordinarily humble and patient, always yielding to His judgment and seeking His will and purpose above our own. Humility is willingness to be an instrument in the hand of God. Humility takes courage and fortitude.
Circumcision was quite a lot to ask of an old man; more still to command his staff of hundreds to follow. No one wants to force others to do something dangerous or painful.
Why did everyone have to be circumcised? This holy laceration of slaves and servants was not an option.
Millennia later, we know that God's purpose for the covenant was never about the land, or the Kings. It was to establish the lineage of His only begotten Son. But Abraham didn't know that, nor could he have understood the purpose of the incarnation. Some information is simply too big to explain.
Abraham truly became the father of billions, more than all the stars of the heavens, through Ishmael, through Isaac, and by adoption through his greatest grandson Jesus. God fulfilled his part of the contract.
Now, back to 99 year old Abraham and 89 year old pregnant Sarah. The plot thickens.