Luke 1:22
And when he came out, he could not speak unto them: and they perceived that he had seen a vision in the temple: for he beckoned unto them, and remained speechless. And when he came out, he could not speak unto them: and they perceived that he had seen a vision in the temple: for he beckoned unto them, and remained speechless.
The vision came while Zacharias stood before the golden altar in the holy place as the cloud of incense with the prayers of Israel ascended before God. The angel stood on the right side of the altar, a position indicating divine favor, though Zacharias took no note of it. It came after many years of prayer for the coming of the Redeemer, with heaven sending its messenger to announce those prayers were about to be answered, but the mercy seemed too great to credit, leaving him filled with fear and self-condemnation.
His long delay inside made the people fear that God was displeased. Because he doubted the message, he was struck temporarily dumb as the penalty for unbelief, so he could only spread out his hands and look toward heaven and could not pronounce the usual benediction. His inability to speak was the sign of the prediction and continued because of his unbelief until the prediction was fulfilled.
When he came out his face shone with light reflected from the angel, the glory of God. From his gestures and continued speechlessness the people perceived that he had seen a vision in the temple. That recognition from silence and beckoning is also shown where the same speechlessness after a vision is met by communicating through gestures.
By signs he told them that an angel had appeared to him and communicated what he had seen and heard. The episode is a parallel for being struck dumb after a vision and communicating by gestures and writing.