Romans 4:19

He regarded not the impotency of his own body, which was as it were dead, because of his age, in respect of any desires or powers of generation. Abraham several years after married Keturah, by whom he had divers children; how then doth the apostle say his body was now dead, or unable for generation? Some say that the deadness of Abraham's body was only in his own opinion. Augustine hath two answers:

1. That his body was not dead simply, but in respect of Sarah; he might be able to beget children of a younger woman.

2. His body was revived, and he received a new generative faculty of God. Another question may be moved, and that is, how the apostle could say that Abraham considered not his own body, being dead; seeing we read, Genesis 17:17, that Abraham, upon the promise of a son, fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? &c. Some answer, that Abraham at first doubted, but afterwards he recollected himself, and got over that unbelief; his faith overcame all difficulties. Others say, that he doubted not at all of the truth of God's promise, but was uncertain only how it should be understood, whether properly or figuratively: see Genesis 17:19. Others say, that these words of Abraham are not words of doubting, but inquiring; they proceed from a desire to be further instructed how that thing should be. It was a question like that of the virgin Mary s, How shall these things be? Augustine says, that Abraham's laughter was not like Sarah s. Hers proceeded from distrust; his, from joy and admiration.