THE DOUBT OF ZACHARY; THE SIGN GIVEN HIM:
The angel said to Zachary: "Thy prayer is heard." Zachary ought to have known by these words that the speaker was a messenger sent by God, for god alone can see the heart, and it was from Him alone that the angel could have known what Zachary had prayed for. Nevertheless, Zachary would not fully believe the good tidings. He and his wife being old, it was so unlikely they should have a son that he asked for a sign. His request was granted. At that very moment he lost both speech and hearing, becoming (according to St. Ambrose, Maldonatus and others) deaf as well as dumb. We shall see in chapter IV that his friends had to make signs to him, because he could not hear them. This deafness and dumbness were meant to serve a threefold purpose:
1. They were to be a sign by which Zachary should know that the angel's message had come from God, and would surely be accomplished;
2. they were a punishment of the holy man's passing doubt, for a willful doubt is a sin. However, we may assume that Zachary's doubt was not quite willful, but rather the result of surprise and want of consideration, and that therefore his sin was only venial;
3. this particular mode of punishment was to be the means of keeping secret the revelation made to Zachary. It had been revealed to him that his son should go before the Redeemer, and that therefore the Redeemer would soon come. Zachary's impulse, in the joy of his heart, would have been to announce to all pious Israelites the good news that the day for which they all longed and the hour of salvation were at hand. Now, however, he was struck dumb, and according to the degrees of God's wisdom, the great secret could not just yet be disclosed to the world.
[From 'A Practical Commentary on Holy Scripture' by Bishop Knecht, D.D.]
(1899 Douay-Rheims Bible)