For this cause it is of faith, that it may be according to grace, to the end that the promise may be sure to all the seed, not to that only which is of the law, but to that also which is of the faith of Abraham, who is the father of us all.

Without being weakened in faith, he didn't consider his own body, already having been worn out, (he being about a hundred years old), and the deadness of Sarah's womb.

that I have great sorrow and unceasing pain in my heart.

For this is a word of promise, "At the appointed time I will come, and Sarah will have a son."