John Calvin Commentary Luke 1:7

"And they had no child, because that Elisabeth was barren, and they both were [now] well stricken in years." — Luke 1:7 (ASV)

And they had no child. By an extraordinary purpose of God it was appointed that John should be born out of the common and ordinary course of nature. The same thing happened with Isaac (Genesis 17:17; Genesis 21:1–3), in whom God had determined to give an uncommon and remarkable demonstration of his favor. Elisabeth had been barren in the prime of life, and now she was in old age, which in itself closes the womb.

By these two hindrances, therefore, the Lord gives a twofold, surprising exhibition of his power, in order to testify, by stretching out his hand, as it were, from heaven, that the Prophet was sent by himself (Malachi 3:1; John 1:6). He is indeed a mortal man, born of earthly parents; but a supernatural method, so to speak, recommends him strongly, as if he had fallen from heaven.