In chapter 17, we see that God starts changing names and declaring specific promises over Abram's life. Abram (99 yrs old at time) is to have a son through Sarai (now Sarah and still 90 yrs old) and they shall call him Isaac. Ishmael (now 13 yrs old) will get circumcised like every other male in his family (which will be a sign in the flesh of the covenant made through a promise). Ishmael will be the father of 12 rulers of a nation. Meanwhile, multiple kings of peoples and nations will come from Isaac and God will renew His covenant through Isaac's line. Got all that? There's actually more but that's good enough for now 🙂
Abraham laughs. Being in God's presence has already put him on the ground but now he laughs because he still can't believe that he will have a child at 100 yrs old while his wife is 90. With all of his strength, there is now way he can make it happen. Yet God's initial statement shines: "I am God Almighty; walk before me faithfully and be blameless." So the Father of all Faith (Hebrews) at the age of 99 starts to learn how to really walk by faith and not by sight or the possibilities of human strength. He chooses to circumcise himself along with every male in his household regardless of age.
Meanwhile, on the way to Sodom, three messengers of the Lord stop by and visit Abraham. While talking, Sarah overhears them say that they'll return in a year and Sarah will have a son by then…She too laughs and then lies about it. She remarks that she's worn out and Abraham is too old.
Abraham was now finding out what true strength really was and that God's promise held stronger than anything he could accomplish under his own strength…he was learning to walk in righteousness. Meanwhile, Isaac was the child of promise who was to father sons of faith…and ultimately a son (descendant boy named Jesus) who was going to bring the fulfillment of God's initial promise.
