Ambrose of Milan
Catena Aurea · Lukasevangelium 1
Then said Mary unto the angel, How shall this be, seeing I know not a man? And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
It was Mary’s part neither to refuse belief in the Angel, nor too hastily take unto herself the divine message. How subdued her answer is, compared with the words of the Priest. Then said Mary to the Angel, How shall this be? She says, How shall this be? He answers, Whereby shall I know this? He refuses to believe that which he says he does not know, and seeks as it were still further authority for belief. She avows herself willing to do that which she doubts not will be done, but how, she is anxious to know. Mary had read, Behold, she shall conceive and bear a son. (Is. 7:14.) She believed therefore that it should be, but how it was to take place she had never read, for even to so great a prophet this had not been revealed. So great a mystery was not to be divulged by the mouth of man, but of an Angel.