1. The encyclical Ad Caeli Reginam proclaiming the Queenship of Mary was promulgated by Pope Pius XII on Oct 11, 1954, near the close of Marian year observed in connection with the centenary celebration of the proclamation of the dogma – Immaculate Conception (1854)
2. Mary is queen in relation to the King the Messiah. Her maternal authority over her royal Son and her care for all Christians give ground and meaning to her own royal title.
3. Jesus the Son of God who humbled Himself to the extent of dying on the cross as a criminal was exalted by God the Father as the King and Kings and Prince of Peace. Likewise Mary humbled herself by saying ‘Behold the handmaid of the Lord’ and thus becomes the Mother of the King and thus the Queen.
4. Christ’s royalty has strong evidence in Scripture. Luke 1:32, He will be great, and will be called the Son of the Most High, and the Lord God will give to him the throne of his ancestor David. Isaiah 9:6, For a child has been born for us, a son given to us; authority rests upon his shoulders; and he is named Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace. Revelation 19:16, On his robe and on his thigh he has a name inscribed, "King of kings and Lord of lords." Thus the Mother of Christ the King, logically becomes the Queen Mother. There are many Church Fathers like John Damascene, Gregory Nazianzen, Jerome who addressed Mary as the Queen.
5. She had her place under the cross because it was God’s will. He wished her to share in her son’s sufferings. He might have spared her; there is no reason why Mary had to live to see her son’s death. If God willed that Mary share her son’s sufferings are we not to conclude that she, the handmaid accepted and fulfilled God’s will perfectly.
‘Because the virgin Mary was raised to such a lofty dignity as to be the mother of the king of kings, it is deservedly and by every right that the Church has honoured her with the title of Queen’ St. Alphonsus Liguori
From the article, ‘Queenship of Mary’ by Aver Johannes in Theology Digest (1955)
Encyclopedia of Mary – Monica and Bill Dodds