Genesis 17:19

Call his name Isaac. “Indeed” — unquestionably — Sarah would become a mother; there was no reason to doubt. The name God selected for Sarah’s promised son, Isaac, means “he laughs.” This may be a reference to Abraham’s laughter, the lasting reminder of a weak moment of doubt. More likely, however, it reflects the joy Abraham would experience with the birth of the son of promise. He might laugh now in doubt, but then he would rejoice indeed, when faith should meet reality (see Gen. 21:6, 7; Isa. 54:1; Gal. 4:27). The names of both Isaac and Ishmael were selected prior to birth, and the names of Abram and Sarai were changed because a new experience was in store for them.