That is an important phrase, “Let it be…” It takes us back to creation itself, when by the word of God, all things came into being.
In the beginning, God said “Let there be light,” and there was. God brought into being everything there was – by His word there came into being all of creation, including man himself. In fact, creation itself is the larger context for the Annunciation to the Blessed Virgin Mary.
As God spoke His creative word in the beginning, so in our remembrance of the beginning of the Incarnation we call to mind Mary’s words, “Let it be…. Let it be unto me according to thy word.” The Virgin Mary’s words, “Let it be,” echo God’s words, “Let there be.” It is, in a way, the continuation of creation and the beginning of our salvation. God says, “Let there be…” and his word brings forth creation; Mary says, “Let it be,” and her words bring forth the Incarnate God into the world.
We beseech thee, O Lord, pour thy grace into our hearts: that, as we have known the Incarnation of thy Son Jesus Christ by the message of an Angel; so by his Cross and Passion we may be brought unto the glory of his Resurrection; through the same Jesus Christ thy Son our Lord, who liveth and reigneth with thee, in the unity of the Holy Spirit, ever one God, world without end. Amen.
Pictured: "The Annunciation" by Arthur Hacker, 1892