Thirteen silent years pass, and God appears again — El Shaddai, "God Almighty" — with new names for both of them. Abram, "exalted father," becomes Abraham, "father of a multitude," before the multitude exists. Sarai becomes Sarah. God rewrites their identities around a promise that, from the outside, looks absurd — a ninety-nine-year-old man is now required to introduce himself as "father of nations." And then the sign: circumcision, cut into the very organ of generation, marking the reproductive future of the whole line as belonging to God. This is worth noting for the same reason chapter fourteen's political detail was worth noting — it's a bodily, transmitted, empirically observable practice carried by every male descendant for millennia, not an abstract symbol. Paul, in Romans four, makes a point of the sequence here worth not skipping past: circumcision is the seal of a righteousness Abraham already had by faith thirteen years earlier, in chapter fifteen. The order is the whole argument — faith first, sign after, so that the sign never becomes the ground of the standing.
And then Abraham laughs — falls on his face and laughs at the promise, in disbelief, to God's own face — and the text doesn't edit that out. C. S. Lewis's instinct is right here: Scripture records its patriarch's undignified doubt without shame, and God simply proceeds anyway, converting that same laugh into the child's own name a few chapters later. That's worth remembering the next time your own doubt feels disqualifying.