1 And the Lord visited Sarah as he had said, and the Lord did unto Sarah as he had spoken. 2 For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him. 3 And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac.
The author of this passage wanted to make it extremely clear that God had made good on His promises. Look at the phrases “as he had said,” “as he had spoken,” and “at the time of which God had spoken.” God had come through completely, and this is an important difference between God and man: God not only keep His promises, He keeps them in every detail of how, when, and where.
Abraham and Sarah’s patience had been tested by this promise. It was years between when God first announced they would have son and the day that it actually occurred. But when God finally gave a specific timeframe for the birth to occur it happened “at the time of which God had spoken.”
Many times we might struggle to believe in God’s promises when it seems long since the time that they should have been fulfilled. Many times God gives us encouragement far in advance of the realization, and it requires great patience to see the journey through to the end. But as soon as God does give specific details of His promises, they will all be met to the letter.