Whereby shall I know this? - The thing was improbable, and he desired “evidence” that it would take place. The testimony of an “angel,” and in such a place, should have been proof enough; but people are slow to believe the testimony of heavenly messengers. As a consequence of not believing, he was struck mute.
Related Commentaries of Luke 1:18
Luke 1:18
18 And Zacharias said unto the angel, Whereby shall I know this? for I am an old man, and my wife well stricken in years.