what did Pope Pius XII intend when he wrote, ”having completed the course of her earthly life”? As in the study of Sacred Scripture, the answer lies in context.
the Holy Father taught that our Immaculate Lady died an earthly death before being assumed bodily into Heaven. This belief is stated repeatedly in the text of Munificentissimus Deus.
Here are some examples from Munificentissimus Deus:
Citing Pope Adrian I, His Holiness Pope Pius XII records: Venerable to us, O Lord, is the festivity of this day on which the holy Mother of God suffered temporal death, but still could not be kept down by the bonds of death, who has begotten your Son our Lord incarnate from herself.”
Citing the Byzantine liturgy: As he kept you a virgin in childbirth, thus he has kept your body incorrupt in the tomb and has glorified it by his divine act of transferring it from the tomb.”
Citing Saint Modestus, the Holy Father writes: As the most glorious Mother of Christ, our Savior and God and the giver of life and immortality, has been endowed with life by him, she has received an eternal incorruptibility of the body together with him who has raised her up from the tomb and has taken her up to himself in a way known only to him.”
Pope Pius XII reveal that he believed and intended to show that the Immaculate Virgin Mary did in fact undergo death prior to her glorious Assumption.
What is death?
Death is the separation of the body from the soul. When the soul leaves the body, you are dead.
The common feature of the Dormition icon is Christ holding the tiny soul of Mary depicted as a baby wrapped in white clothes.
The Holy Apostles surround her demonstrating that the Dormition and Assumption had apostolic witness and are part of the deposit of Faith.
But Mary didn’t sin!
It should be stated that Mary did not die because of sin, but rather in her desire to be conformed to Christ in all things – to be the speculum justitiae, mirror of justice.
Her death gave her dominion over Purgatory as prophesied in Ecclesiasticus 24 and gave her more meritorious prayers for those in the hour of death.
