For this is what the promise said: “About this time next year I will return, and Sarah shall have a son.”
Romans 9:9 ESV

It was impossible for Sarah to conceive a child, yet God made it happen. There is no question that Isaac was the genetic offspring of Abraham and Sarah, but he was also a child solely of promise. For obvious reasons, Abraham and Sarah both had to accept God’s offer of a child. Without their cooperation, Isaac could not have been born. Their involvement was necessary at every step in the process.

This fact should undermine the confidence of Calvinists in their interpretation of this passage, but it seems to go completely unnoticed.