“And of the son of the maid I will make a nation also, because he is your descendant.” Gen. 21:13
Sarah has just told Abraham to send Hagar and Ishmael away into the desert to an uncertain fate. Abraham is justifiably distressed; he wants to please Sarah, but does not want to send away his eldest son and second wife. God reaffirms to Abraham that sending Hagar and Ishmael away is the desired course of action, and so God reassures Abraham that not only will Ishmael survive, he will thrive and flourish.
After all, God promised Abraham in Genesis 12:2 that He would make him into a great nation; and that He would make his descendants as the ‘dust of the earth’ (Gen. 13:16), and likewise as the stars of the sky (Gen. 15:5). God also confirmed this promise would apply to children through Hagar (Gen. 16:10).
We tend to forget that part of the great number of Arabs today is due to this promise; we tend to remember this as if it only applied to the Jews, but not so; God never specified which woman Abraham’s descendants would be multiplied through, and God was faithful in keeping that promise, even if Abraham had a lapse of faith as to how God would bring it about.