God chose Mary before the Creation of the world. He sanctified her at the moment she was conceived. She was set apart (made holy) a vessel for His Divine Purpose.
Eph 1:3-5
4 even as he chose us in him before the foundation of the world, that we should be holy and blameless before him."
When the fullness of time came, He sent the angel Gabriel:
Luke 1:28
28 And he came to her and said, “Hail, O favored one, the Lord is with you!”
The Lord was already with her in a special way. She was “full of grace” (kecharitooménee). We know that original sin is the fallen state of man, where we are unable to be filled with grace. This is why the fathers understood this greeting to mean that Mary had no original sin.
Then Mary asks how this will happen, and the angel explains:
Luke 1:35
"The Holy Spirit will come upon you,
and the power of the Most High will overshadow you;
therefore the child to be born will be called holy,
the Son of God. "
In this verse the fathers see that Mary became the spouse of the HS.
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Mary was given a gift, but that does not make her more than Jesus’s earthly mother.
It is not God’s gifts that “make” us more or less, but the grace of God. During Jesus public ministry, the people noted that John the Baptist was great. He was also filled with the HS, and had a powerful ministry. Jesus taught that he had the greatest ministry that had ever been given:
Luke 7:28
28 I tell you, among those born of women none is greater than John; yet he who is least in the kingdom of God is greater than he."
However, because Mary was the Ark of the New Covenant,(and the very first Christian), her ministry was even greater than that of John.
Mary was Jesus earthly mother, but because of the nature of who Jesus is, she is a mother like no other. She is also the mother of the second person of the Trinity, the “God-Bearer”, or as the fathers call her, the Theotokos.
He is flesh of her flesh, and bone of her bone. He would not have drawn from any tainted flesh to make His Temple. This is the great mystery of the Incarnation:
Phil 2:5-8
6 who, though he was in the form of God, did not count equality with God a thing to be grasped, 7 but emptied himself, taking the form of a servant,
being born in the likeness of men. 8 And being found in human form he humbled himself and became obedient unto death"
Scripture tells us that “being Jesus’ earthly mother” greatly blessed Mary. She was considered “blessed among women”, and God “highly exalted” her in her lowly state. She is “the Mother of My Lord” (second person of the Trinity).