Sarah, Abraham's wife, is mentioned in Hebrews 11:11 saying, "By faith even Sarah herself received the ability to conceive, even beyond the proper time of life, since she considered Him faithful who had promised;..."Sarah was not a woman without faults. She had some! Genesis 11:12 tells us she "laughed" when Abraham told her she was going to have a son (Genesis 11:12). When we look back to Hebrews 11:11 where scripture tells us, "she (meaning Sarah)considered Him faithful who had promised." The Greek meaning for the word, faithful in Hebrews 11:11 means 'worthy of belief.' So we know that Sarah believed God ultimately( for its recorded in Hebrews) despite her doubtful laugh that God would bring her a son in her old age.
Genesis 21:1 says, "Then the LORD took note of Sarah as He had said, and the LORD did for Sarah as He had promised." The birth of Isaac reminds us that God surely does keep His promises. He brought about the possibility for Sarah to give birth to Isaac. Sarah had nothing to do with it. It was impossible for her, but not for God.
God is a rewarder of those who seek Him. Abraham and Sarah despite their faults were still able to be used by God because they believed HIM. Their reward was a son. Our reward, was given through the birth of Isaac, for the lineage of Jesus would come through him. Because of Jesus, we could be born again through His death, burial and Resurrection. As the bible says,"For He who promised is Faithful." God can be trusted.
Believe God IS (not was) and that He will reward those who wholeheartedly seek Him.
Warren Wiersbe says from his commentary(The Wiersbe Bible Commentary,published by David C.Cook, 2007; pg.83) "God keeps His promises and gives you the power you need to do what He wants you to do. No matter how long you may have to wait, you can trust God to accomplish His purposes."Praying for you all today that you will trust who God is and that you would seek Him with your whole heart.