Theophylact of Ohrid
Catena Aurea · Lukasevangelium 1
And there appeared unto him an angel of the the Lord standing on the right side of the altar of incense. And when Zacharias saw him, he was troubled, and fear fell upon him. But the angel said unto him, Fear not, Zacharias: for thy prayer is heard; and thy wife Elisabeth shall bear thee a son, and thou shalt call his name John. And thou shalt have joy and gladness; and many shall rejoice at his birth.
As if when Zacharias asks, How shall I know this? the angel answers, Because Elisabeth shall bring forth a son, thou shalt believe that the sins of thy people are forgiven.