Genesis 17:18 - Ellicott's Commentary On The Whole Bible

O that Ishmael... — For thirteen years Ishmael had been the “son of the house” (Genesis 15:3), and regarded probably as the true heir. Mingled then with Abraham’s joy there was also the pain, natural to a father, of knowing that this transference of the promise to Sarah’s child meant the deposition and disappointment of one who for so long had held the post of honour. Stoicism would have repressed this upright and natural feeling, but God hears and accepts the father’s prayers; and while the birthright and religious pre-eminence is justly given to the son of the freewoman, there is a large earthly blessing for the handmaid’s son.