John Gill 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 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 spoke 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.