At the beginning of St. Thomas Aquinas' excellent — but poorly known — work on the Blessed Sacrament entitled, The Bread of Life OR On the Adorable Sacrament of the Altar, the Angelic Doctor provides us with three reasons why Our Lord instituted the Sacrament of His Body and Blood through the Holy Mass:
There are three reasons for the institution of the Blessed Sacrament: A, the memory of Our Saviour; B, the Sacrifice of the Altar; C, the food of man.
In regard to the first, St. Thomas teaches that sin causes us to forget God:
We often draw away our minds and all our senses from God, and wander far from Him, by evil thoughts and disordered pleasures; but by the grace of this Holy Sacrament, we learn to turn our thoughts from all that is evil and give ourselves altogether to God.
This "memory of Our Saviour" is kept alive within our soul by three powerful reminders of Christ's love for us, all of which are made present through the Blessed Sacrament. The Holy Sacrament of the Altar recalls to our minds (1) Our Lord's forgiveness of sins, (2) His powerful redemption which frees us from bondage, and (3) the unending depths of His goodness towards us.
In regard to the second reason for Christ instituting the Blessed Sacrament (the Sacrifice of the Altar), St. Thomas offers for our reflection a quote from Pope Alexander:
Nothing in the sacrifices of the Church can be greater than the Body and Blood of Christ. This is before all other oblations. It must be offered to God with a pure conscience and taken with a pure mind. As it is greater than all sacrifices, so it is more adorable.
In order to show the greatness of the Blessed Sacrament as the most perfect oblation to God, we are invited to consider three aspects of the Eucharistic Sacrifice:
the power of the Author of this Sacrifice (Jesus Christ);
the immensity of the debt that we contracted towards God through our sins; and
the insufficiency of the sacrifices of the Old Testament Law to atone for our sinful deeds.
Sin is an offence against God. And because it is an offence against the One who is infinite in holiness, it requires an infinite atonement: that is, an infinite "paying of the debt." Since only one who is infinitely holy can do this, it required nothing less than God Himself to descend from Heaven and become incarnate, so that He Himself could offer the only Sacrifice capable of making such an atonement: the Sacrifice of Himself in His human nature (Body, Blood, and Soul). And the Blessed Sacrament contains this very Sacrifice itself.
The third reason for Our Lord instituting the Blessed Sacrament was for it to become the Food of man. In the words of Aquinas:
This is the medicinal Food against the corruptions of the death-bringing apple. Now this corruption, brought by Adam and Eve on the human race, was so deep-seated that it would have been incurable but for that wondrous medicine which only the wisdom of God could make. St. Ambrose says, ‘The Body of Christ is that spiritual medicine which, tasted with reverence, purifies those that are devoted to it.’
Thus the Blessed Sacrament is medicinal Food for the soul, and particularly for the human soul tainted by the Original Sin of Adam and Eve. Some of the consequences of this Original Sin (which still remain after the sin itself is washed away in the font of Holy Baptism) include (1) darkness or ignorance in the soul; (2) evil desires in the body (i.e. concupiscence of the flesh); and (3) death for both the soul and the body. Saint Thomas proposes that the Blessed Sacrament is a medicinal Food because it provides a triple remedy to this threefold disorder, for the Holy Body of Christ:
enlightens the darkness of the soul;
heals the evil desires of the body (flesh); and
destroys the power of death over both the body and the soul.
And to highlight this point (namely, that the Body of Christ received worthily in Holy Communion has the power to bring about this triple remedy), Saint Thomas concludes his reflection with a beautiful quotation from the Church Father St. Hilary of Poitiers:
When we have eaten the Flesh of the Lord and drunk His Blood, then we are in Him and He in us. Christ dwelling by His Flesh in our bodies is the cause of our life, for He truly is life. We shall live by Him, as He lives by the Father, who is in Him.
Thus, in summary, the three primary reasons given by the Angelic Doctor why Our Lord instituted the Blessed Sacrament of the Altar are:
to remind us of the goodness of Our Saviour in freeing us from the bondage of sin;
to provide us with access to the one Sacrifice capable of making atonement and paying the debt for our sins; and
to feed us with medicinal Food that heals both our body and our soul, and thus frees both from the bonds of eternal death.
This last point is why the Church calls the Blessed Sacrament the Medicine of immortality and the Food of the Angels, which has become the Food of men.
Panis angelicus fit panis hominum!