1 Now Sarai, Abram's wife had borne him no children, and she had an Egyptian maid whose name was Hagar. 2 So Sarai said to Abram, "Now behold, the LORD has prevented me from bearing children. Please go in to my maid ; perhaps I will obtain children through her." And Abram listened to the voice of Sarai. 3 After Abram had lived ten years in the land of Canaan, Abram's wife Sarai took Hagar the Egyptian, her maid, and gave her to her husband Abram as his wife. 4 He went in to Hagar, and she conceived; ... 15 So Hagar bore Abram a son ; and Abram called the name of his son, whom Hagar bore, Ishmael. 16 Abram was eighty-six years old when Hagar bore Ishmael to him." (Genesis 16:1-4, 15-16 NASB)
1 Now the LORD appeared to him by the oaks of Mamre, while he was sitting at the tent door in the heat of the day. 2 When he lifted up his eyes and looked, behold, three men were standing opposite him; and when he saw them, he ran from the tent door to meet them and bowed himself to the earth, 3 and said, "My Lord, if now I have found favor in Your sight, please do not pass your servant by. 4 "Please let a little water be brought and wash your feet, and rest yourselves under the tree ; 5 and I will bring a piece of bread, that you may refresh yourselves ; after that you may go on, since you have visited your servant." And they said, "So do, as you have said." 6 So Abraham hurried into the tent to Sarah, and said, "Quickly, prepare three measures of fine flour , knead it and make bread cakes." 7 Abraham also ran to the herd, and took a tender and choice calf and gave it to the servant, and he hurried to prepare it. 8 He took curds and milk and the calf which he had prepared, and placed it before them; and he was standing by them under the tree as they ate. 9 Then they said to him, "Where is Sarah your wife ?" And he said, "There, in the tent." 10 He said, "I will surely return to you at this time next year ; and behold, Sarah your wife will have a son." And Sarah was listening at the tent door, which was behind him. 11 Now Abraham and Sarah were old, advanced in age ; Sarah was past childbearing . 12 Sarah laughed to herself, saying, "After I have become old, shall I have pleasure, my lord being old also?" 13 And the LORD said to Abraham, "Why did Sarah laugh, saying, 'Shall I indeed bear a child, when I am so old ?' 14 "Is anything too difficult for the LORD ? At the appointed time I will return to you, at this time next year, and Sarah will have a son." 15 Sarah denied it however, saying, "I did not laugh "; for she was afraid. And He said, "No, but you did laugh." (Genesis 18:1-15)
1 Then the LORD took note of Sarah as He had said, and the LORD did for Sarah as He had promised. 2 So Sarah conceived and bore a son to Abraham in his old age, at the appointed time of which God had spoken to him. 3 Abraham called the name of his son who was born to him, whom Sarah bore to him, Isaac. ... 5 Now Abraham was one hundred years old when his son Isaac was born to him. 6 Sarah said, "God has made laughter for me; everyone who hears will laugh with me." 7 And she said, "Who would have said to Abraham that Sarah would nurse children ? Yet I have borne him a son in his old age." (Genesis 21:1-3, 5-7 NASB)
There were a couple of problems with this plan that God had. At least, from Abraham and Sarah's perspective there were. The question they continued to ask, for more than a a decade after they followed God to Canaan was, "How is Abraham going to be the father of many nations if his wife couldn't even have one child?"
They were old, and getting older, and there had been no baby for them. As time goes by Sarah gets it into her head that maybe she isn't intended to be part of fulfilling the promise. Maybe, for God to keep his promise to Abraham, she needs to get out of the way. Maybe another woman was supposed to bear the promise and give Abraham children. Abraham thinks she might be right, so he takes Hagar as a wife too. (You could do that kind of thing back in those days. It wasn't as weird as it would be now.) Hagar does bear Abraham a son. He's not the son God had planned, but God takes care of her and Ishmael anyway and blesses them.
But God has his own plans, and Sarah, the one who thinks it's impossible for her to be a part of them herself, figures right into the center of it. She laughs to hear the promise delivered, because the thought is too ridiculous. But you will find, as we go through this story, that God often chooses the most unlikely people to accomplish his plans. It's not ridiculous at all that someone like you, or me, or even Sarah could be part of God's plan for healing the world, and finishing the work.
The things that seem hard to us aren't hard for God at all.
Can you list all the reasons that Sarah had to think that it was impossible for her to have a baby?
She had never had a baby, even when she was young. She couldn't have babies. Something was wrong in her body.
She was very old now, too old to have children even if everything worked properly. Her husband was very old now too, almost 100! He was too old to father children.
Sarah had good reasons to laugh, and to doubt.
But God gave her a baby anyway. A little boy named Isaac, which means laughter. Now Isaac too was part of God's plan to bless all of the earth. He was the way that God would keep his promise to Abraham.