What does Genesis 17:19 mean?
The previous verse showed 99-year-old Abraham's reaction to God's revelation: that his barren, 89-year-old wife, Sarah, would bear him a son after all. Abraham's immediate response was disbelieving laughter (Genesis 17:17).Beyond laughter, Abraham's first coherent thought was for his 13-year-old son, born through his servant Hagar: "If only Ishmael might live under your blessing!" Now God responds with a "yes but." God will reveal in the following verses that He will bless Ishmael. However, Ishmael will not be the child of the covenant promises God has given to Abraham. Instead, that fulfillment would come through Sarah and their previously inconceivable son, who God commands to be named Isaac. It was with Isaac and his offspring that God would establish His everlasting covenant.
This name is not without a purpose. Isaac means "he laughs." Abraham's first response to the idea of having a son with his 90-year-old wife was to laugh. Sarah, too, will laugh at the idea (Genesis 18:12). Their son's name will serve as a permanent reminder of how God can fulfill promises which our limited experience says are impossible (Mark 10:27).