Fully convinced that God is able ...
Even when there was no reason for hope, Abraham kept hoping — believing that he would become the father of many nations. For God had said to him, “That’s how many descendants you will have!” And Abraham’s faith did not weaken, even though, at about 100 years of age, he figured his body was as good as dead—and so was Sarah’s womb. Abraham never wavered in believing God’s promise. In fact, his faith grew stronger, and in this he brought glory to God. He was fully convinced that God is able to do whatever he promises.
The older I get the more that I am aware of how crazy the birth of Isaac was. Babies are for young people. These days it is rare for a woman to give birth after forty. Yet God visited Abraham in his old age and promised him a son. Even so, the promise took years to come to pass. While he waited he fathered a son with his wife's maid - Abraham's waiting was not perfect. Yet somehow Abraham continued to believe the thing that God promised him that he would do.
I believe that there are differences between this specific promise and the promises that we often find in the scriptures. In fact, this promise is pretty rare in biblical text. Most of the promises we read of in scripture are more general in nature. Yet these promises are acquired the same way that Sarah conceived Isaac - by faith and faithfulness. We must not only believe God today but continue to believe him tomorrow. And sometimes the promise is fulfilled in heaven.
We are convinced Lord that you are able to do what you have promised to do.