Ambrose of Milan
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.
It is well said that there appeared an angel to Zacharias, who suddenly beheld him; and this is the expression especially used by Divine Scripture with respect to angels or God, that what cannot be seen beforehand may be said to appear. For things which are the objects of our senses are not seen as He is seen, Who is seen only as He will, and Whose nature is not to be seen.