John Calvin Commentary Luke 1:64

"And his mouth was opened immediately, and his tongue [loosed], and he spake, blessing God." — Luke 1:64 (ASV)

And his mouth was instantly opened: God puts honor on the birth of his prophet by restoring speech to his father, for there can be no doubt that this benefit was delayed until that day with the express object and design of fixing people's eyes upon John. Zacharias spoke, blessing God.

He did so not only to testify his gratitude but also to inform his relatives and neighbors that this punishment had been inflicted on him because he had been too slow to believe: for he was not ashamed to unite his own dishonor with the praises of divine glory. Thus, it became universally known that the child's birth was not an accidental or ordinary event but had been promised by an announcement from heaven.⁶⁵

⁶⁵ “Mais selon la promesse expresse de Dieu, qui avoit este apportee et revelee par l'ange.” — “But according to the express promise of God, which had been brought and revealed by the angel.”.” — “But according to the express promise of God, which had been brought and revealed by the angel.”