Hebrews 11:18 - Of whom it was said, That in Isaac shall thy seed be called:
Genesis 18:14 - Is any thing too hard for the LORD? At the time appointed I will return unto thee, according to the time of life, and Sarah shall have a son.
The birth of Isaac as a Divine miracle is demonstrated by the facts that:
1. Both Abram and Sarai were old; Genesis 17:17
2. Sarai was barren -- unable to bear children -- during her childbearing years; Genesis 11:30
3. Sarai was past childbearing; Genesis 18:11
Romans 4:16, 17 - ...Abraham; who is the father of us all, (As it is written, I have made thee a father of many nations,) before him whom he believed, even God, who quickeneth the dead, and calleth those things which be not as though they were.