Because he was of the house and lineage of David (Luke 2:4).
The book of the sacred Gospels referring the genealogy to Joseph, who was descended from David’s house, has proved through him that the Virgin also was of the same tribe as David, inasmuch as the Divine law commanded that marriages should be confined to those of the same tribe.
And the interpreter of the heavenly doctrines, the great apostle Paul, clearly declares the truth, bearing witness that the Lord arose out of Juda.
The natures, however, which combined unto this real union were different, but from the two together is one God the Son, without the diversity of the natures being destroyed by the union. For a union of two natures was made, and therefore we confess One Christ, One Son, One Lord. And it is with reference to this notion of a union without confusion that we proclaim the holy Virgin to be the mother of God.
Because God the Word was made flesh and became man, and by the act of conception united to Himself the temple that He received from her. For we perceive that two natures, by an inseparable union, met together in Him without confusion, and indivisibly. For the flesh is flesh, and not deity, even though it became the flesh of God.
And in like manner also the Word is God, and not flesh, though for the dispensation’s sake He made the flesh His own.
But although the natures which concurred in forming the union are both different and unequal to one another, yet He Who is formed from them both is only One. Nor may we separate the One Lord Jesus Christ into man severally and God severally, but we affirm that Christ Jesus is One and the Same, acknowledging the distinction of the natures, and preserving them free from confusion with one another.
With Mary, his betrothed wife, being great with child (Luke 2:5).
The sacred Evangelist says that Mary was betrothed to Joseph, to shew that the conception had taken place upon her betrothal solely, and that the birth of the Emanuel was miraculous, and not in accordance with the laws of nature.
For the holy Virgin did not bear from the immission of man’s seed. And what was the reason of this? Christ, Who is the first-fruits of all, the second Adam according to the Scriptures, was born of the Spirit, that he might transmit the grace (of the spiritual birth) to us also.
For we too were intended, no longer to bear the name of sons of men, but of God rather, having obtained the new birth of the Spirit in Christ first, that he might be “foremost among all,” as the most wise Paul declares.