Albert Barnes Commentary Luke 1:64

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

His mouth was opened. That is, he was enabled to speak. For nine months he had been mute, and they probably supposed he had been afflicted with paralysis and would not recover. Therefore, they were amazed when he spoke. For one act of disbelief, all this calamity had come upon him, and it had not come without effect. With true gratitude, he offered praise to God for the birth of a son and for his restoration to the blessings of speech.