Ambrose of Milan — as excerpted in the Catena Aurea on Luke 1:11-14

“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.”