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 till that day with the express object and design of fixing the eyes of men upon John. Zacharias spake, blessing God He did so, not only for the purpose of testifying his gratitude, but 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 with his own dishonor the praises of the divine glory. Thus it became universally known, that the birth of the child was not an accidental or ordinary event, but had been promised by an announcement from heaven. [1]

Footnotes

1 - "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."

His mouth was opened - That is, he was enabled to speak. For nine months he had been mute, and it is probable that they supposed that he had been afflicted with a paralytic affection, and that he would not recover. Hence, their amazement 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.

The latter clause of the preceding verse should be joined with the beginning of this, as follows: And they marveled all, for his mouth was opened, etc. Every person must see the propriety of putting this clause, And they marveled all, to the beginning of the 64th verse, instead of leaving it at the end of the 63rd, as in the common version. The people did not wonder because Zacharias said, He shall be called John; but because he himself was that instant restored to the use of his speech.
And he spake, and praised God - In his nine months' silence, he had learned the proper use of his tongue; and God, whose power was discredited by it, is now magnified. Happy they who, in religious matters, only break silence in order to speak of the loving-kindness of the Lord!

And his mouth was opened immediately,.... As soon as ever the child was named, and so all things accomplished which the angel had foretold,
and his tongue loosed; the impediments of speech were removed, and the use of his tongue and lips was restored unto him:
and he spake and praised God; for the safe delivery of his wife; for the birth of his son, the forerunner of Christ; for the conception of the Messiah; for God's gracious regards to his church and people, in these instances; and for the restoration of speech and hearing to himself, of which he had been some time deprived for his unbelief.

mouth opened immediately--on thus palpably showing his full faith in the vision, for disbelieving which he had been struck dumb (Luke 1:13, Luke 1:20).