34 As We have already mentioned, Venerable Brothers, according to ancient tradition and the sacred liturgy the main principle on which the royal dignity of Mary rests is without doubt her Divine Motherhood. In Holy Writ, concerning the Son whom Mary will conceive, We read this sentence: "He shall be called the Son of the most High, and the Lord God shall give unto him the throne of David his father, and he shall reign in the house of Jacob forever, and of his kingdom there will be no end," and in addition Mary is called "Mother of the Lord"; from this it is easily concluded that she is a Queen, since she bore a son who, at the very moment of His conception, because of the hypostatic union of the human nature with the Word, was also as man King and Lord of all things. So with complete justice St. John Damascene could write: "When she became Mother of the Creator, she truly became Queen of every creature." Likewise, it can be said that the heavenly voice of the Archangel Gabriel was the first to proclaim Mary's royal office.
Source: Ad Caeli Reginam (Vatican.va)