With whom did God establish His covenant, Isaac or Ishmael?
God established His covenant with Isaac, the son Sarah would bear, not with Ishmael. In Genesis 17:19 God tells Abraham that Sarah shall bear a son called Isaac and confirms His covenant with him. Though Ishmael is not cast off as nothing, the covenant continues through Isaac.
Les explains that Genesis 17 adds another part to the Abrahamic covenant. Abraham, who was a hundred years old, laughed and naturally asked God why He could not use Ishmael, since Abraham already had him. But God said Sarah, who was ninety, would bear a son to be named Isaac. God declared, "I will establish my covenant, not with Ishmael, but with Isaac." Les notes, however, that God was not casting off the people of Ishmael as nothing. This also becomes intrinsic to a later verse in Romans 9.
And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant, and with his seed after him.