Explanation had from the Guardian Angel:
from the diary of Gemma Galgani
On 25th March, in the morning, Jesus was felt by my soul more than usually: I was on my own and nothing could distract me; towards noon, I feel my Guardian Angel touching me on a shoulder and he says: "Gemma, I come on behalf of Jesus to fulfil his promise". I did not know what I would have to think; I was astonished to hearing those words. "Daughter", added, "I am your Guardian Angel, sent from God; I will make you to understand the greatest mystery".
My wonder increased, I still did not understand... Then my angel told me: "Do you remember what I promised you twelve days ago?". I thought and I remembered soon. "Be aware, my daughter, that I will tell you about the Most Holy Mary, of a young girl so humble towards the world, but so great in front of God; I will tell you about the most beautiful, the most holy of all the creatures, of the most beloved daughter of the Very High, of her who was destined to the unparalleled dignity of mother of God".
... I was late in the night, and very Holy Mary was in Her room alone: She prayed, completely captured by God. Suddenly a great light enters that poor room, and the archangel, with human aspect, submitted and majestic in the same time, and surrounded by numbers of angels, goes close to Mary. It greets Her like a lady, smiles as it is taking good news, and with sweet words it tells Her: "Hail, Mary, God is with you. You are the blessed of all the women". Beautiful, great and sublime greeting, which was never been heard before and never will be heard again!
... "After saying these words, the celestial archangel stood waiting for her to ask it to explain its divine embassy. But Mary was upset and She did not say any word and thought. But do you think my daughter that the angels have never come to Mary before? She enjoyed their visit and their sweet talks any moment... She does not wonder the mysterious meaning, but She is upset because She thinks herself unworthy of such Angelic salute. Ah! My daughter", it repeated to me, "if Mary had known how much God liked Her humility, She would not have estimated Herself unworthy of the honour of an angel. "Why", She wondered, "an angel of God calls me full of grace, and I feel myself unworthy of any divine favour? Why", thought Mary on Her own, "an angel of the paradise calls me blessed among the women, and I am the most useless, the vilest, the basest of the women? Which mystery hides under the veil of such a lofty salute?"
"Mary did not answer the salute of the angel; then Gabriel in order to stop her fear repeats: "Do not be afraid, Mary, you have found grace towards the Most High. From this moment you will conceive in your womb a son, you will call him Jesus, and He will be called Son of the Most High by all people: He will be given the throne of David, He will reign eternally, and His reign will never end". With these sublime words the archangel explained its embassy to Mary completely...
"The angel had manifested to the Virgin the mystery of its great mission that is that She was going to become mother of the Son of the Most High. But She told the angel: "And how can it happen, if my virginal candour is not cancelled? "(Isaiah had already foretold, that Christ would be born by a virgin mother)... You must know, told me the angel, "that very Holy Mary, with an example never heard, from Her tender years had consecrated Her virginal flower to the celestial spouse of the chaste souls.
"Think", told me, "that very Holy Mary said nothing about the great mystery, but She talked and was worry about Her pure candour, and asked that angel of God... Have you understood, daughter, how much Mary loved this beautiful, angelic, celestial virtue? But who, do you think, loved it more? Jesus or Mary? Surely Jesus, who would never, chose a not pure, not immaculate mother.
The angel Gabriel answered: "Mary, the Saint Spirit will come to you, the sublime virtue of the Most High will shade you; and the true Son of God will be born by you. At this point I tell you that Elisabeth, your relative, although very old, has conceived a son, and she is already in the sixth month: she was said sterile, but, remember, noting is impossible to God". The angel Gabriel goes on with these words: "Be sure and consoled, blessed Virgin: the divine spirit will come and fertilise your immaculate womb. The omnipotent virtue of the Most High will do a new wonder in you, so that the virgin honour will be saved and, in the same time, you will become mother. The saint that you will conceive in your womb will not be other than the Son of God". With these words the archangel Gabriel revealed and explained the mystery, reassured Mary.
"Everything was explained now; just the last word of Mary was required so that the virgin became mother of God. The divine Word, generated by the Father in the splendour of the Saints, could not have a father in earth, as well as He had no mother in Heaven. And Mary, as She had been elected mother of the Only-Begotten of the divine Father, became the only-begotten daughter of the same Father. As She had to give human limbs from her virginal substance to the divine Word, She was raised to the dignity of mother of the Son of God. As Mary was that woman, who the Saint Spirit would shade with His omnipotent virtue so that She would become virgin mother of a God son, She was therefore raised to the honour of spouse of the Saint Spirit.
"After Explaining the mystery and reassuring the virgin, the divine messenger was waiting for Her answer, that is the consent of Mary to the incarnation of the eternal Word... and She answers: "Here is the servant of the Lord, shall everything happen according to your word". The great accent is said, Mary is the mother of the Son of the Most High. Heaven enjoy these words, the entire world is consoled. The respectful angel bows down before Her, and then comes back to Heaven.
By accepting the very high dignity of mother of God, Mary called herself servant of the Lord humbly. That great humility remained unchanged at the glorious salute and at the much more glorious proposal to become mother of the divine Word.
"Mary said the prodigious Fiat and, in a moment, a tender and a perfect little body was formed by the divine Spirit in Her womb of Her very pure virginal substance, and a human soul was joined, perfectly combined, the divine Person of the Word. Or miracle! That God, who cannot be contained in the amplitude of the universe, is closed in the womb of Mary. That God, who holds the universe with a finger, is held by the pure womb of a virgin. Who can says therefore which fullness of joy overflowed the soul of Mary in that happy moment, in which She became mother of the Son of God? The King of the Kings, the great Gentleman of powerful men has placed His throne in the womb of Mary. An infinite joy overflowed Mary, when She looked at the infinite light and could see the hidden splendours of the divinity.
"By accepting the incomparable dignity of mother of God Mary accepted the generous commitment of mother of the humanity. Letís enjoy ourselves: Mary, giving the angel Her assent, has adopted you as sons, She has become the mother of everybody".