This is quite possibly one of the most self-evident and easiest doctrines of the Church to defend and explain. This is because it follows necessarily from faith in Christ.
When we think of the word "communion" we might think of several things. We may think of the Eucharist, we may think of juice and cracker packets, we may think of unity, solemn prayers, etc. Depending on who we are, the word communion can be rather nebulous. Rarely do people stop and look at the obvious etymology of the word: common-union. Some common union must be effected in order for communion to be effected. We must participate in that common union to be affected by the communion.
Now, the Catholic dogma known as the "Communion of the Saints" states that the Church Militant (the Church alive on Earth) benefits from the intercession of the Church Triumphant (the Church in Heaven) and the Church Purgative (those in Purgatory) benefits from the intercession of both the Church Triumphant and Militant. These three have a common union with each other because they are commonly unified in Christ by the Holy Spirit. That is to say, they are each of them individually and all of them communally unified with God the Father and God the Son, having been incorporated by the gift of the Holy Spirit who is God.
All true Trinitarian theology states that there is one God, three Persons. Further, it is stated that the Holy Spirit is the mutual love that God the Father has for God the Son and visa versa. Therefore, the Holy Spirit while being God, is the union between God the Father and God the Son. So, it follows necessarily that the Holy Spirit is equally God. There is no disputation or discrepancy on this point between Christians, regardless of whether they be from the Orthodoxies in the East, the Roman Church in the West and those in communion with her, or even amongst the Protestants. If anyone denies these essentials they deny what Christianity has faith in and are not Christians, ipso facto.
The purpose of bringing this up is to show, or remind rather, what kind of union it is that the Christian is incorporated into. The union takes on the attributes of the unified; for this reason God became a man. St. Gregory of Nazianzus said," That which is not assumed is not redeemed." God shared in our humanity so that we might share in His divinity. Being human ourselves we know precisely what it means for Him to take on humanity.
But what does it mean to put on divinity? Does the Catholic faith teach that we become gods independent from the One True God? By no means. It teaches that we share in what He promises, namely His Holy Spirit. So then, what does it mean to share in this divinity? It means that we will be holy as He is holy, it means that we share in His eternal life, in His omnipotence, in His omnipresence. It means that we enjoy that same degree of closeness with the Trinity that It enjoys Itself. This is accomplished by the work of Christ, at the good pleasure of the Father, by the indwelling of the Holy Spirit.
Now, it is written in the Book of Hebrews that we have been given an eternal priesthood, in the order of Melchizedek and that we have a High priest, who is Christ. Christ is both the Lamb who was sacrificed and the Priest who sacrificed Himself for us and we know this from the Gospels, because he told them that no one takes His life away from Him, but He lays it down for His sheep and has the power to take it up again. And He tells us further that this command is from His Father.
Now, there is no other purpose for a priest except for to make sacrifice for the people, but here we see that Christ the high priest has made a sacrifice once and for all. And yet our priesthood does not end here; no, much to the contrary, we hear that ours is an eternal priesthood. What does this mean? Let us examine. God is infinite and God was sacrificed, therefore the sacrifice is infinite. We behold an infinite sacrifice of mercy, more than sufficient for the offenses of mankind. But why then are we yet priests?
St. Paul says," How I wish that I were with you to make up for that which is lacking in the wounds of Christ!" Does St. Paul contradict what we know to be true, which is precisely what we just stated, that Christ's sacrifice is more than sufficient for the sins of mankind? By no means does he disagree, but understand what he means to say... you and I are the wounds of Christ. Not only this, but we must assist one another and workout our own salvation, meeting faith with works and giving life to the hope we have in Him, participating with the work of Christ.
If then we are the Body of Christ, which is the Church, and we are the wounds of Christ shouldn't we know that we have become once with the Eternal Sacrifice and the same Priest who did sacrifice It? Aside from all this, don't priests also make offerings of prayer? Isn't it written that our praise and prayers go up to heaven as an offering of sweet smelling incense? We offer these prayers and praises not only for ourselves, but for each other and the whole world, because God wills that all men shall be saved according to the scriptures.
We read in the Gospel that after Christ was done answering the Sadducees concerning their question about marriage and the resurrection He gave them evidence of the resurrection and proof of the life of the righteous. He said unto them that when God spoke to Moses in the bush He said," I am the God of Abraham, Isaac, and Jacob." And Christ said that God was a God of the living and not of the dead.
So, what is this that you say that the dead are cut off from the living? Who told you this and where did you read it? Did you forget that Moses and Elijah were present at Christ's transfiguration? How then do you define eternal life? If you say that the soul lives, where? If the soul is present in the body the body is alive, and if the soul is absent from the body the body is dead. And if the soul is alive the body shall live; this is the resurrection.
If then you have eternal life and an eternal priesthood, don't you know that you will not cease your priesthood in bodily death, seeing that the soul yet lives? Or do you believe that you are only a priest sometimes, and that you only kind of have eternal life? Eternal means one thing only, there is no such thing as delimited eternal life. Either it's eternal or it's not. There's no such thing as a delimited eternal priesthood; you either participate eternally in an eternal priesthood or you don't.
So, what's your priestly duty after you have died? Will it not surely be to pray for those who toil on the earth against temptation to evil and the power of darkness? Will you not pray for those souls which fly toward God and experience a painful purging, which is called Purgatory?
Do you suppose you will pray as feebly as you do, now? Ignorant of what is needed, deluded by vanity, swayed and distracted by the flesh, deceived by sin? Or will you, standing in the presence of God, after having come through the fire, not pray more perfectly? Will you not confess what we already know to be true, which is precisely what we said earlier, that we share in the divinity of Christ just as He shares in our humanity? Will you not enjoy His omnipotence, His omnipresence, His wisdom, His compassion, His knowledge, His understanding? You must already know that you will!
And wouldn't you prefer someone to be praying for you in this manner? When you communicate to these saints who have gone on before you, how will they hear you? Will your prayer be carried on the wind into heaven to souls that do not have bodies, and therefore have no ears? Don't you know that you rather will communicate by a common union, namely by the Holy Spirit? Are you confused, even though you know that you are One in Christ and both have the same Holy Spirit, and share in the same eternal life and are members of the same priesthood, with One High Priest? This is the 'Communion of the Saints.'
Now, without a doubt, there are many of you who until this point are saying that Christ alone is the Mediator between God and man. You are right, but here you show more than a little confusion. We know that He is in us and we are in Him. Further, we know that we are the Body of Christ and that we are His wounds. Then, why are you so fearful as though this mediation is not done in Christ? You yourselves just now said that Christ is the Mediator between God and man. Listen to your own mouth, friend, and be taught by yourselves that the doctrine of the Holy Catholic and Apostolic Church is true.
And if there is anyone else who says that they shall have none pray for them but Christ alone, to you I say charitably and gently, do not be a hypocrite. You are the same person who asks his fellows to pray for him, you are the one who says," I covet your prayers." You are the one who when distressed looks for the one who seems the most spiritual among you and ask him to pray for you; very often it's your pastor. Therefore, again, do not be a hypocrite concerning the Church and the things of God.
The communion of the saints is a blessing to us. The Saints in heaven, those martyrs and Apostles, innocents and holy, are not bound as they once were by the flesh, praying for one person at a time. They now pray perfectly and intimately for you and me and for all, without ceasing. This is the will of the Father, established by Christ, accomplished by the Holy Spirit. Christ has prepared this for you and set it before you on the table. Christ said that whoever rejects those whom He sends also rejects Him; and whosoever rejects the Son rejects the Father also, because He and the Father are One. It is not right to reject Him. "Today, if you hear the call of the Lord, harden not your heart."
"Every art and every inquiry, and similarly every action and choice, is thought to aim at some good; and for this reason the good has rightly been declared to be that at which all things aim." ~Aristotle~