What does Genesis 17:17 mean?
Compared to modern times, it seems the people of Abraham's era lived slightly longer lives. But even then, 100-year-old men did not have babies with 90-year-old women. The very idea of it was laughable. So Abraham fell facedown before God and laughed to himself. It's notable that Abraham's disbelieving comment is directed to himself. Abraham is, very directly, expressing doubt that God can or will follow through on this particular declaration.It's not that Abraham had stopped believing that God would keep His promise to give him countless offspring. He just didn't expect those offspring to come through Sarah. They waited a full decade between receiving the promise and deciding that God must mean for a child to come through another woman (Genesis 16:1–2). That had worked, according to Abraham and his wife. Hagar birthed Ishmael, now 13 years old (Genesis 16:16). God told Hagar that Ishmael's offspring would be so numerous as to be uncountable (Genesis 16:10–12). Apparently, Abraham seems to have decided that Ishmael's birth was God's plan to fulfill His promise.
It was not. God said Sarah would become the mother of nations. Abraham laughed and then, in the following verse, protested. This, in part, explains the specific name God chooses for this son of promise: Isaac, which means "he laughs."