Genesis 17: It's Sarah and Isaac

Contrast the human plan of the last chapter with the divine plan of this chapter. El Shaddai (God Almighty) never invites or encourages people to sin. His covenants can be kept without  any immorality on our part. So, He changed the names of Abram and Sarai, told them to name their son Isaac, and caused a miraculous conception. El Shaddai, indeed.