And God said to Abraham, “As for you, you shall keep my covenant, you and your offspring after you throughout their generations."
God keeps giving a hope of a promise to Abraham. He tells the patriarch that next year he and Sarah will have a son together. And he renews a covenant with a commitment asked of Abraham. Circumcision would be the sign of obedience and faith in this covenant with God.
So now Abraham, who is 99, and Ishmael his son through Hagar, who is an adolescent of 13, are both circumcised. Without being too graphic, I have to say that this was a real step of obedience. An extreme old man and a young teenage boy both obey God in this matter. I can't help but speculate on how well that must have went. It's hard to get a teenager excited about anything spiritual... let alone to have an elderly man perform a circumcision on him! Yet Abraham and his household obeyed the LORD in this matter.
Abraham's faith is now fully displayed by his obedience to the LORD's command. The covenant was given and followed. Now the old man and his old wife will patiently wait for God to keep His covenant in the birth of Isaac. The promise of God has now been a sacrificial and painful commitment of faith by Abraham. The cost of that faith shall soon be rewarded many times over in the joy of a son for Abraham and Sarah.