The Body of Christ
The Body of Christ was originally formed in the womb of the Blessed Virgin Mary. Christ grew, and was born, and grew some more, just like we do. At the Last Supper, He instituted the sacred priesthood of the New Covenant and gave us His Body and Blood in the Blessed Sacrament. Having completed His work of salvation and ascended in His human body into heaven, He nonetheless remains bodily on earth in the Blessed Sacrament.
As Pope Pius explained, however, this physical body of Christ is not separate from the Church of Christ. In fact Christ is the Head of a Body, His mystical Body. All baptized men, women, and children are members of this body, the Corpus Christi mysticum.
In the encyclical, Pope Pius considers this phrase one word at a time.
The Church is one body in numerous ways. For one thing, just like a person is sick if any part of his body is sick, likewise the Church: If one part of the Church suffers, the whole Church suffers. Or, to look at it the other way: If you or I won a gold medal at the Olympics–no matter what event–the medal would belong equally to the hands, the feet, the knees, the nose—the whole body, not just one part of the body. Likewise, just as the Mystical Body of Christ shares in the grace and glory of God as a whole, every particular member of the Body shares in this same grace and glory.
Another sign of bodily unity is eating and drinking. We continue living precisely because we are able to replenish our entire bodies by using particular parts: The mouth eats, the stomach digests, but the whole body receives nutrition. The Church also has a source of replenishment and growth, namely the grace of the sacraments, and this single source nourishes the entire mystical Body.
The bodily unity of the Church is both invisible and visible. The Church is united by a common faith in the invisible mysteries of God, and we cannot see the inner life of the Church, namely the Holy Spirit. At the same time, the mystical Body is visibly united by the one leader, chief shepherd, and teacher–the Pope. It is impossible to be united fully to the invisible realities of faith and the Holy Spirit without being united visibly with the Pope, and vice-versa.
So the Church is one Body, but it is not just any body—it is the Body of Christ. Christ is our Head. The incarnate Word of God did not come into the world just for the fun of it; He came to share His grace with the human race.
Again, this union of the mystical Body with her divine Head is both invisible and visible. Christ dwells in our souls by invisible grace, a grace which we believe in but cannot see. Nonetheless, this invisible grace is given to us through the visible signs of the Church, especially the sacraments. Through the sacraments, the invisible Christ acts, using His visible ministers as His instruments, like a surgeon wielding a scalpel or a sculptor using a chisel. Whenever the Gospel is proclaimed, or whenever anyone is baptized, or whenever the holy sacrifice of the altar is offered to the Father and Holy Communion is given—all of which are visible actions—it is in fact the invisible High Priest Jesus Christ Who does these things.
And just as the invisible and visible aspects of the Church’s bodily unity are inseparable, the visible and invisible aspects of our union with Christ are also inseparable. There is only one way (that we know of) by which we can infallibly receive grace, and that is the sacraments. On the other hand, if we receive the visible sacraments without faith in their unseen power, then they won’t do us any good. Receiving a sacrament without faith in it is like putting diesel fuel in the tank of a car that runs on unleaded.
Lastly, the Church is the mystical Body of Christ. We are not the Body of Christ physically, because we are not God. We do not cease to be our individual selves by being members of the Church. On the other hand, neither is the Church simply an association, a moral body, like the Marine Corps, or a baseball team, or a legislative ‘body’. The Church is brought together by something greater than the individual will of all the members. The Church is united as one by the power of God; we form, therefore, a transcendent society, a mystical Body, and the soul of this unique body is the Holy Spirit.
When we worship the Blessed Sacrament, we hail the verum Corpus Christi, the Body, Blood, soul and divinity of our Lord Jesus Christ. We worship and adore the divine Head of the Body.
At the same time, we look forward in hope towards the everlasting day when Christ will be all in all, when the totus Christus, divine Head and mystical members, will be perfectly united, to the glory of the Almighty Father.