By faith Sarah herself also received strength to conceive seed, and she bore a child when she was past the age, because she judged Him faithful who had promised. Hebrews 11:11 NKJV
God promised Abraham and Sarah a son. He promised to give them descendants as numerous as the stars. This seemed physically impossible since Sarah was past the age of child bearing. Abraham was one hundred years old and Sarah was ninety. They trusted in God’s faithfulness.
Yes, their faith may have waivered when they took matters into their own hands with Hagar. Sarah thought her maid, Hagar, could give Abraham the promised Child, but that was not God’s plan.