Moussar sur La Genèse 17:21

Abraham, as the progenitor of Isaac, who had removed the foreskin which Adam had been gross enough to "pull down," had managed to become the person who is fit to be the carrier of the מרכבה, the Presence of G–d, as is clearly indicated in Genesis 17,22 "G–d ascended from Abraham." This ascent occurred after G–d had concluded an eternal covenant with him, as is specifically stated here ואת בריתי אקים את יצחק, "and I will maintain My covenant with Isaac, whom Sarah will bear for you." This is a promise that in the future the world will achieve its perfection, as always planned by G–d. In view of the above, the meal at which the timing of Isaac's birth was announced was certainly an event of spiritual significance, something not unlike the meal G–d is said to serve the righteous in the future.