How was Isaac born to Abraham and Sarah?
Scripture teaches that Isaac was born to Abraham and Sarah miraculously by God's promise, not by human effort. Though both were past the age of childbearing, God gave them Isaac after about twenty years of waiting. Paul contrasts him with Ishmael, who was born of the bondwoman according to the flesh.
Les explains that in Galatians 4, Paul addresses Gentile churches in Asia Minor, reminding them they are not under the Law but under grace. The Judaizers were insisting Gentiles must embrace Judaism and keep the Law. Paul uses Abraham's two sons as an allegory: Hagar, the Egyptian slave girl, had Ishmael, born after the flesh; Sarah, Abraham's legitimate wife, had Isaac. Ishmael's birth was Sarah's idea, not God's. But God had promised Isaac for over twenty years. Abraham and Sarah patiently waited, and even when they were past the time for childbearing, God miraculously gave them Isaac.