Assumption of the Virgin, 1670 Bartolome Esteban Murillo.
There's a saying that nothing is inevitable except death and taxes. Some folks are even able to avoid paying their taxes—but nobody seems to be able to cheat death.
The reason for this is simple: We are complex beings, made up of innumerable atoms, and molecules, and intricate body parts—a vast material assembly, held together in a organized fashion by a spiritual soul. But the nature of material things is that tend to become disorganized, disintegrate, and fall apart. We see this in nature all the time: Even very well made things eventually wear out and break down; plants and animals as well as people eventually give up the ghost and die. As the Psalmist says, "70 is the sum of our years, or 80 if we are strong."
When God created the material universe, He did so knowing its transitory nature. Presumably, He made it this way so that it could renew itself over the ages, with new creatures being born out of the elements of old ones.
But in creating human beings, God made creatures who were in many ways like Himself. He made us, as we say "in His image and likeness." The most important similarity we have to God is that we possess an immortal soul—a spiritual entity which will never be subject to material decomposition.
And to go along with this immortal soul, God gave Adam and Eve the very special grace of also preserving their bodies from the ravages of accident, disease, suffering, and death. Had they retained their original sinless innocence, their material bodies might well have functioned perfectly forever. "Grace," as we say, "perfects nature," and an absolutely sinless soul would have held the body together indefinitely.
But, we know that Adam and Eve disobeyed God—they sinned—and they lost all of these preternatural graces given by God.
Yet, we also know that at the time of God's choosing, He sent a woman into the world, who would be the Mother of His Divine Son. He determined that His Son would take flesh of her flesh, and bone of her bone. Now, since the entirely sinless flesh of Jesus was the flesh of Mary, God determined also that He would preserve Mary from every stain of sin—right from the very moment of her conception.
And Mary, being the perfectly humble and obedient daughter that she was, persevered in the will of the Father, and remained equally sinless throughout her entire life.
It is for this reason that we celebrate the feast of the Assumption—for it has been the unchanging belief of Christians, since the first centuries, "that the immaculate Mary ever Virgin, Mother of God, when she had finished the course of her life, was taken up body and soul into the glory of heaven."
These words are from the dogmatic definition of Pope Pius XII in 1950, but in essence, they have always been the constant doctrinal teaching of the Church. As Pope Pius pointed out in his accompanying encyclical, Munificentissimus Deus, this teaching comes to us from the earliest traditions of the Church, is found in the earliest liturgies of both the Greek and Roman churches, in the innumerable church buildings dedicated under the title of the Assumption, or Dormition, to use the Greek term. It is implicit in the definition of the Council of Ephesus, that Mary is the Mother of God; and in Pope Pius IX's definition of the Immaculate Conception. It is found in the writings of the Fathers and Doctors of the Church, as well as in those of the Popes over the centuries.
There is a little room for discussion in this. We may ask whether Mary died at all—"Dormition" means a "falling asleep," and may indicate that Mary was preserved not just from the decay of the grave, but from death itself. Some theologians believe this was the case. Others suggest that our sinless Mother offered herself up to death, just as her sinless Son—something that she didn't have to do, but did anyway in union with our redemption. In any event, we know that because of her sinless life, both her body and soul were carried to heaven at its end.
This feast of the Assumption should be one of great rejoicing for all Christians. Like the Resurrection and Ascension of our Lord, it proclaims the victory of the human race over sin, suffering, and death.
In a sense, it is even more significant for us, in that Mary was a mere human, as we are. This shows us that it is possible for us mere humans to live a life free from sin.
Mary could have sinned, but she chose not to, and was rewarded with eternal life of body and soul. We have sinned, but we can confess our sins, and follow Mary's lead. Because of original sin and our own actual sins, we will not be assumed into heaven in our bodies—but, by virtue of the Redemption which Mary made possible, we can look forward to a glorious resurrection at the end of time.
Because of Mary, we can look forward to the life of the world to come.