Praised be Jesus and Mary, now and forever.
Today, we celebrate the dedication of St. Mary of the Angels, often considered the oldest church dedicated to Mary in the West. It was dedicated by Pope St. Sixtus III around A.D. 435. This was four years after the Council of Ephesus defined the dogma that Mary is the Mother of God, thereby vindicating a title long used by the faithful. This title is also important because it affirms the unity of Christ. This means that when Jesus died on the cross, the person who died was the Word of God, and that made His death sufficient to save us and a thousand worlds besides. When Sixtus did this, there was already a church on that place, which Pope Liberius had built about 70 years earlier in a place called the Sechenium. It's not clear what exactly the Sechenium was. Some think it was a large hall to which Liberius added an apse to make it a church, but it could have just been a square or a street where there was space to build. St. Sixtus rebuilt the church and dedicated it to the honor of the Mother of God, decorating it with mosaics. The mosaic over the apse has the inscription, "Bishop Sixtus, for the people of God." In a floor mosaic, he wrote a short poem. The last traces of this mosaic disappeared at the beginning of the 17th century, but by then there were many copies of the poem. It said, "Virgin Mary, to you I, Sixtus, dedicated a new building, a tribute worthy of your salvation-bearing womb. You, a mother who did not know man, you brought forth our salvation while your womb remained intact. Behold, those who testified about your womb bring you trophies. Under the feet of each lies whatever he suffered—iron, flame, beasts, a river, and cruel venom. Yet one crown awaits these several deaths."
The inscription refers to mosaics on the walls that depicted two processions of martyrs moving toward the altar. One reason these decorations are appropriate is that these martyrs bore witness to the faith, which includes the fact that Christ was born of the Virgin Mary, as the ancient baptismal creed of Rome says. The Pope rightly began the poem by calling Our Lady, “Virgin Mary,” as the Apostle's Creed does. Another reason is that it was grace that made them capable of bearing martyrdom without denying the faith. Before Sixtus was elected Pope, St. Augustine wrote him a letter about grace, in which he said, "What else but His gifts does God crown when He crowns our merits?" So they owe the merit of their martyrdom to the grace of God, and they receive that grace through Mary's mediation, which St. Cyril of Alexandria had extolled at the Council of Ephesus. 
So the martyrs processed toward the altar of Mary's church, bringing the trophies of their victories, that is, the instruments of death over which they triumphed through her mediation. At least some of these martyrs had probably invoked Mary as the Mother of God. An Egyptian papyrus from around the year 250 contains the prayer Sub Tuum Praesidium in Greek. This is the prayer that in English begins, "We fly to thy patronage, O Holy Mother of God." This was the prayer of persecuted Christians, at least in Egypt, when Christianity was illegal. It may well have been prayed in other regions as well, where the climate was not so gentle to ancient manuscripts.
So Mary is Mother of God, or that Mary is Mother of God, is a truth of the faith, but a truth especially important for the confidence that it gives us in the power of her intercession. She is the mediatrix of all graces, as popes have said many times. Grace comes down from heaven like snow, softly and silently falling upon us. Martin Luther liked this symbol, but saw grace, unfortunately, as merely covering our sins to hide them from God, as a pile of manure can look beautiful after it snows. But grace really does change us inside. St. Peter says that it makes us partakers of the divine nature, and St. Paul says of himself, "the inward man is renewed day by day." Grace is like snow in that it is from above, coming down from the Father of lights. In scripture, says St. Lawrence of Brindisi, snow is a symbol of holiness and purity. Daniel saw God, so to say, clothed with snow. His clothing was snow white. At Christ's transfiguration, too, His face shone like the sun and His clothes became white as snow. Christ appeared to John in the Apocalypse with the hair of His head white as snow. The angel who announced Christ's resurrection appeared clothed with snow. His appearance was like lightning, and his clothing white as snow. Jeremiah says that Zion's young men were once brighter than snow. The psalmist again refers to a soul sanctified by God and restored to its baptismal innocence. "Wash me, make me whiter than snow." And the Lord says through Isaiah, "though your sins be like scarlet, they may become white as snow."
So it is fitting that a medieval legend associates St. Mary Major with snow. The story is that the Virgin Mary appeared in a dream to the patrician John on August the 4th and told him to build a church on the site where he would find snow the following morning. In the morning he went to Pope Liberius and told him of his dream. The Pope believed him because he, too, had dreamed that it would snow and the Virgin Mary had told him to lay out a church where the snow fell. It does not snow often in Rome and snow in August would certainly be a convincing sign from heaven, especially if it was foretold by a dream. In memory of this story, on this day, a snowfall of jasmine petals is dropped from the ceiling of the basilica at the Gloria during the main Mass and again at Vespers during the Magnificat. This story is very fitting, for the Immaculate Conception of Mary is like a snowfall in the dog days of summer. It's something that nature cannot possibly produce. It brings refreshment to mankind and marks the beginning of the salvation brought by Christ. Like the snow, it comes from above and brings purity. Mary is so full of grace that it is almost as if she had become grace. St. Lawrence says, "today's miracle reveals Mary with a purity unmatched below God Himself." White is whiter than snow. She is purer than the angels and archangels in cherubim and seraphim. Her purity and sanctity are astonishing beyond belief. Whiteness like snow in the soul consists in perfect observance of God's laws. Only sin can stain the soul and smudge the purity of conscience.
So today we are shown Mary's purity and reminded of her mediation, which can make us pure and give us the strength to choose even death rather than sin. If the martyrs overcame all their many diverse torments by the grace that they obtained from her, we can overcome anything by her mediation. May the snow of grace refresh us when the heat of passions threatens and help us to become holy and immaculate like our Blessed Mother. 
Praised be Jesus and Mary, now and forever.