The Blessed Virgin Mary is called by many different names in the Catholic Church, with many of them featured in the Litany of Loreto.
One title in particular was explained to St. Gertrude the Great, who saw it revealed to her in a vision.
According to the 19th-century book The Spirit of St. Gertrude, Mary appeared to Gertrude in the form of a lily.
The following day as Gertrude prayed, the Mother of God appeared to her in the presence of the ever adorable Trinity under the form of a white lily with three leaves, one standing erect and the other two bent down.
She then understood this title more fully and came to know its symbolism.
Symbolism of the Lily of the Trinity
By this she understood that it was not without reason that the Blessed Mother of God was called the white lily of the Trinity, since she contained in herself with more plenitude and perfection than any other creature, the virtues of the Most Holy Trinity, which she had never sullied by the slightest stain of sin.
The upright leaf of the lily represented the omnipotence of God the Father and the two leaves which bent down, the wisdom and love of the Son and the Holy Spirit, to which the Holy Virgin approaches so nearly.
St. Gertrude would frequently call upon the Virgin Mary under this title with the following prayer.
Hail, White Lily of the ever peaceful and glorious Trinity! Hail, effulgent rose, the delight of heaven, of whom the King of heaven was born, and by whose milk He was nourished! Do thou feed our souls by the effusions of thy Divine influences.
The Virgin Mary said to St. Gertrude, "I will appear at the hour of death to those who salute me thus in such glory that they will anticipate the very joys of heaven."
