3Abram fell facedown, and God said to him, 4“As for me, this is my covenant with you: You will be the father of many nations. 5No longer will you be called Abram; your name will be Abraham, for I have made you a father of many nations" (Genesis 17:3-5).
After all the years of waiting for God’s promises to be fulfilled, imagine what it was like when God visited Abram and told him that Abram and Sarai’s names were changed to mother of princes and father of many. Abraham was now 100 years old and Sarai 90. Imagine what it was like to tell their friends who went to war with them, Mamre the Amorite, and Eshkol and Aner (Genesis 14:13) of their name change at such an age. Do you think they were ridiculed? Nowadays we would call that “eccentricity” or even madness. Here are the verses that relate to the impossibility of having a son at such an age:
15God also said to Abraham, “As for Sarai your wife, you are no longer to call her Sarai; her name will be Sarah. 16I will bless her and will surely give you a son by her. I will bless her so that she will be the mother of nations; kings of peoples will come from her.” 17Abraham fell facedown; he laughed and said to himself, “Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?” 18And Abraham said to God, “If only Ishmael might live under your blessing!” 19Then God said, “Yes, but your wife Sarah will bear you a son, and you will call him Isaac. I will establish my covenant with him as an everlasting covenant for his descendants after him (Genesis 17:15-19 Emphasis mine).
Inside the tent and away from view, Sarah laughed within herself (Genesis 17:12) when she heard the LORD say that she would be a mother at ninety years old. She thought of the impossibility of it all, a woman of ninety worn out with age and long past childbearing ability, and Abraham being ten years older than her, how could this be possible? She utters not a sound, but the Lord knew her thoughts. In Sarai’s hearing, He said to Abraham:
“Why did Sarah laugh and say, ‘Will I really have a child, now that I am old?’ 14Is anything too hard for the LORD? I will return to you at the appointed time next year, and Sarah will have a son.” 15Sarah was afraid, so she lied and said, “I did not laugh.” But he said, “Yes, you did laugh” (Genesis 1813-15).
God knew the thoughts of Sarah, and He knows our thoughts too. We have a theological word for this ability of God to know all things; we say that God is omniscient. He knows all things and in all creatures at all times, either in the past, present, or future. God knows them all perfectly. God's knowledge is absolute and not acquired. He never has to learn anything.
29Are not two sparrows sold for a penny? Yet not one of them will fall to the ground apart from the will of your Father. 30And even the very hairs of your head are all numbered. 31So don’t be afraid; you are worth more than many sparrows (Matthew 10:29-31).
A.W. Tozer has spoken of God’s omniscience in this way:
"God knows instantly and effortlessly all matter and all matters, all mind and every mind, all spirit and all spirits, all being and every being, all creaturehood and all creatures, every plurality and all pluralities, all law and every law, all relations, all causes, all thoughts, all mysteries, all enigmas, all feelings, all desires, every unuttered secret, all thrones and dominions, all personalities, all things visible and invisible in heaven and in the earth, motion, space, time, life, death, good, evil, heaven, and hell."
There was a gentle rebuke about Sarah’s laugh of unbelief. How would you feel about hearing from a total stranger that you were going to have a baby next year when you have been waiting for a baby all your married life? Oh yes, and now that it is physically impossible, you hear that it is going to happen at the age of ninety! Sarah realizes that this person called the LORD (YaHVeH) has heard her thoughts while she was in her tent—her laugh was not audible, yet He spoke of her future and the dream she and Abraham had waited for all this time! To realize that God knows her every thought is also to know that this same God can do anything. She is told by the LORD, "Is anything too hard for the Lord?" (v. 14). Whatever you are dealing with, know this; the Lord understands your frustration, unbelief, and your struggles, and is aware of your thoughts. It does not stop Him from being able to provide the answer. Keith Thomas
 A.W. Tozer, The Knowledge of the Holy (New York, Harper and Row, 1961) p.63.