On November 1, Holy Mother Church celebrates throughout the world the Church Triumphant: that is, all the Angels and Saints currently enjoying the Beatific Vision of God in the Kingdom of Heaven (Yes, All Saints' Day includes the Angels, as the traditional texts of the sacred liturgy — both from the Mass and the Divine Office — so clearly show). This joyful celebration is, in fact, a celebration of God's mercy and goodness which is so great that it overflows into His creatures. The holiness of the Saints is a splendid reflection or mirror of the holiness of Christ Himself, and, indeed, of the Most Holy Trinity — the Source and Origin of all holiness. There are few feast days in the Catholic liturgical calendar that express as much joy and jubilance for Holy Mother Church than the feast of All the Saints. This is why it is a Holy Day of Obligation in many countries (sadly, not here in Canada).
But the almost infinite, overflowing exhilaration of November 1's feast day of All the Saints gives way on the following day (November 2) to the mournful, sombre mood of the Mass of All Souls' Day. We move from yesterday's focus on the Church Triumphant (Angels and Saints in Heaven) to the sobering reality of the Church Suffering — those holy souls that are still in Purgatory and in need of final purification before they can be admitted to the Heavenly Court and bask in the holy presence of God for all eternity.
Why do we call these souls "holy souls" if they are still in need of purification? The main reason is because they have left this world in a state of grace: they have died in friendship with God, without any unconfessed mortal sins to contend with. However, they still have need of being fully cleansed of any disordered attachments to sin that may linger in their soul even after death. Often, they also have not made sufficient amends or reparation for their sins, though the sins themselves have been confessed and are therefore forgiven by God.
The Universal Church has always encouraged the members of the Church Militant here on earth to pray and offer sacrifices for the souls in Purgatory. It is an act of great charity like no other. Our prayers and our sacrifices can obtain merit for these holy souls and, when united to the infinite merits of the Cross of Our Lord Jesus Christ (which is the source of all merit), they can alleviate their sufferings and hasten their final entry into Heaven.
We read in the Second Book of Maccabees, Chapter 12 (Douay-Rheims Bible, Challoner revision):
38 So Judas having gathered together his army, came into the city Odollam: and when the seventh day came, they purified themselves according to the custom, and kept the sabbath in the place.
39 And the day following Judas came with his company, to take away the bodies of them that were slain, and to bury them with their kinsmen, in the sepulchres of their fathers.
40 And they found under the coats of the slain some of the donaries of the idols of Jamnia, which the law forbiddeth to the Jews: so that all plainly saw, that for this cause they were slain.
41 Then they all blessed the just judgment of the Lord, who had discovered the things that were hidden.
42 And so betaking themselves to prayers, they besought him, that the sin which had been committed might be forgotten. But the most valiant Judas exhorted the people to keep themselves from sin, forasmuch as they saw before their eyes what had happened, because of the sins of those that were slain.
43 And making a gathering, he sent twelve thousand drachms of silver to Jerusalem for sacrifice to be offered for the sins of the dead, thinking well and religiously concerning the resurrection,
44 (For if he had not hoped that they that were slain should rise again, it would have seemed superfluous and vain to pray for the dead,)
45 And because he considered that they who had fallen asleep with godliness, had great grace laid up for them.
46 It is therefore a holy and wholesome thought to pray for the dead, that they may be loosed from sins.
This passage of Sacred Scripture is one of the Biblical foundations for the Catholic Church's practice of praying and offering sacrifices for the dead. Any prayer, any sacrifice offered in love and in a spirit of atonement for the dead is good. Many Catholics pray rosaries, novenas, and other prayers for the holy souls. Some even offer up bodily sacrifices on their behalf (fasting, mortification, etc) — which is even better. But there is one prayer, one sacrifice that surpasses all the others in terms of its efficacy in alleviating the sufferings of the souls in Purgatory and hastening their entrance into Heaven: the Holy Sacrifice of the Mass.
While some Masses have as their primary intention the souls in Purgatory (and it's always good to offer Masses specifically for this intention), nonetheless each and every Mass is offered for all the living and all the dead — that is, for the entire Church Militant and Church Suffering. There is not one Mass that does not bring solace to the suffering souls in Purgatory. In the Roman Canon (which traditionally was always prayed silently by the celebrant priest at the Altar), the priest pauses for a moment of reflection at two different times: once before the Consecration and once after. These two moments are known respectively as the Memento of the Living and the Memento of the Dead. Memento here means commemoration. The first commemoration is before the Body and Blood of Christ are sacramentally present on the Altar; the second, after they have already been consecrated and are truly present there.
The Church's Trinitarian Structure
God reveals Himself through Divine Revelation as a Trinity of Three Holy Persons united in one Divine Nature. The Catholic Church, which Our Lord founded to be the instrument of God's presence and graces in this world, must, therefore, reflect this Trinitarian nature of God. And indeed, so it does! By way of analogy (which, it must be noted, is always an imperfection comparison), the three parts of Holy Mother Church reflect the three Persons of the Holy Trinity.
The Church Triumphant is analogous to God the Father. The Father is the Source of all glory, all splendour, all holiness. As the unengendered and unspirated member of the Most Holy Trinity, the Father is often referred to in Catholic theology as the "Beginning without beginning" or the "Origin without origin." Of course, all Three Persons of the Blessed Trinity are eternal. However, the Person of the Son comes from the Father through the process that we call begetting or engendering, and the Person of the Holy Ghost comes from the Father and the Son through their communal and personal act of spirating. So only the Father is neither engendered nor spirated from any of the other Persons of the Most Holy Trinity. He is therefore the Source of all sources, the Origin of all origins. So too at the level of glory. God the Father is the Source of divine glory and triumph. Thus the Church Triumphant in Heaven is a reflection of the First Person of the Most Holy Trinity.
The Church Suffering is analogous to God the Son. It is God the Son, the Second Person of the Most Holy Trinity, who became incarnate, coming into this world and becoming man so that He may offer His life for us on the Cross in order to merit our eternal salvation. The Passion and sufferings of Christ over the course of His earthly life, and the fact that, through the Incarnation, He became capable of suffering and death (something which God in His divine nature is incapable of), is the reason why the Church Suffering in Purgatory is analogous to God the Son.
And finally we have the third part of the Church founded by Jesus Christ: the Church Militant. This consists of the members of the Church who are still here on earth, militating (fighting) for virtue and holiness, and for the glory of God and the salvation of souls. This part of the Church is analogous to the Holy Ghost, the Third Person of the Most Holy Trinity. The Holy Ghost is the one who strengthens us in our battle against temptation and sin. Although holiness comes from all Three Persons of the Godhead (every action of God outside Himself is the action of all Three Persons together), Catholic theology often attributes our sanctification to the Third Person of the Trinity. He is called, in the Creed, Dominum et Vivificantem (literally: the Lord and Vivifier). To vivify means literally "to make alive." The Holy Ghost is the "soul of the Church" in the sense that it is He who gives the Church divine life. That life is given to the Church in order to keep her on the straight and narrow path so that she, in turn, can guide her children on that same path to eternal salvation.
The Catholic Mass Unites the Three Parts of the Church
All the above shows how the tripartite structure of the Catholic Church (Church Triumphant, Church Suffering, and Church Militant) splendidly reflects the Trinitarian nature of God Himself. But where do we see these three parts of Holy Mother Church united in the most perfect way here on earth? The answer: in the Catholic Mass! Holy Mass is where we encounter the fullness of unity between the Church Triumphant in Heaven, the Church Suffering in Purgatory, and the Church Militant here on earth.
At every Mass, the Church Militant is present (even in a "private Mass" with only one priest and one server). At every Mass, the entire Heavenly Court is present as the Church Triumphant worships God not only in Heaven but also wherever Christ becomes truly present through the Holy Eucharist here on earth. And at every Mass, the Church Suffering is present too, since all Masses are offered for the souls in Purgatory and bring consolation and solace to the suffering souls.
Thus it is the eternal Sacrifice of the Cross, made present through the Holy Sacrifice of the Mass, that unites the Church in Heaven, in Purgatory, and on earth. Perhaps this is one reason why the Holy Eucharist has been righly been called the "Source and Summit" of our Catholic Faith.