Why is Isaac called a son of promise and a son of God's miracle-working power?
Isaac is called a son of promise because God promised Abraham a son named Isaac and would not let His promise fail, and a son of miracle-working power because both Abraham and Sarah were far beyond normal childbearing age, making his birth miraculous.
Les teaches that God's promise to Abraham would not fail even though Abraham was nearing 100 and Sarah nearing 90. God said He would give them a son and they would call his name Isaac, and His covenant would be established with Isaac, whom Sarah would bear at the set time in the next year. Les explains that Isaac was a son of promise, and on top of that a son of the miracle-working power of God, because both Abraham and Sarah were way beyond normal childbearing.