When Abraham was 99 yrs. old, the LORD appeared to him the second time (Gen. 17:1). He re-affirmed his earlier promise to Abraham (Gen. 12). Abraham will be a father of many nations. God will give Abraham many children. God said that the son shall come through Sarah.
Abraham knew about it, but not Sarah. So God visited Abraham at the oaks of Mamre. God wanted Sarah to know and believe in God’s promise.
Abraham was resting in his tent in the heat of the day one day. Suddenly, three divine visitors appeared before him. He was so excited that he offered them food and water. He told Sarah to prepare bread, milk, and a young calf.
While eating, they asked Abraham, “Where is Sarah?” Abraham said, “She’s inside the tent.” Then the LORD said, “I will surely return to you about this time next year, and Sarah your wife shall have a son” (Gen. 18:10). In that the LORD shall return about that time the next year indicates that the LORD was present with Abraham in the form of the three divine visitors. Most likely, the three divine guests were the three Persons of the Trinity in physical form.
All the while Sarah was listening to their talk. When Sarah heard that she will have a son, she laughed. She was already 90 yrs. old—too old to bear a son. But the LORD heard Sarah laugh. So He asked, “Why did Sarah laugh?” Then He added, “Is anything too hard for the LORD?” (v. 14).
Sarah, acting like a “Denial Queen,” said, “I did not laugh.” But the LORD told her, “No, but you did laugh” (v. 15).
The question is a question about divine capability. It is as if God is asking, “Why do you doubt my capability?” The reason the LORD asked that question is because He was challenging the faith of Sarah.