Romans 9:9: For This Is What The Promise Said: "About This Time Next Year I Will Return, And Sarah Shall Have A Son."
 For this is what the promise said: “About this time next year I will return, and Sarah shall have a son.” (ESV)
We can read this verse and wonder, what is so special about the promise that Sarah would have a son. Sarah was barren. She was in her nineties. The timing seemed to be too incredible to even believe. The name Isaac literally means "laughter." This is due to the fact that Sarah laughed when she heard the promise of God that she would become a mother.
Consider the parallel here. The promise of a child who would come and bring salvation to the world was given to the people of Israel. All of the prophets spoke of the Christ who would come, the Messiah. Sarah did not truly believe that the promise of God would come about. She laughed. Many Jews did not believe that the promise of God would come about either.
Abraham was told of the specific time that God had established that Sarah would have a son. This promise was completely of God and God was to get all of the glory. No one in the world would have believed that Sarah could have become a mother at her age, but the promise was fulfilled just as God had said, at exactly the time that God said it would happen.
Consider the promise of the Christ being born. When would it come? Galatians 4:4 says, "But when the fullness of time had come, God sent forth his Son, born of woman, born under the law." The promise of God came about, just as God said it would. The timing of it was established by God. He gave what could hardly be believed. He gets all the glory.
God's word is solid. When God makes a promise he fulfills it. When you read of the promises of God in Scripture be encouraged. He will fulfill his Word. He will do it, and he will get all the glory.