Thou shalt be dumb, and not able to speak,— The affirmation of a thing joined with the denial of its contrary, is an idiom peculiar to the Jewish language, and is the strongest affirmation possible. The style of St. John is remarkable for the frequency of this idiom. See on Luke 1:22.
Related Commentaries of Luke 1:20
Luke 1:20
20 And, behold, thou shalt be dumb, and not able to speak, until the day that these things shall be performed, because thou believest not my words, which shall be fulfilled in their season.