What does the word "communion" mean? When someone "communes" with nature, they are going outside to be present in the world. This is different from going outside just to walk across the street. They actually want to be present and aware of the things around them--natural things, not man-made things. They want to be particularly observant and enjoy nature: a cool breeze, warm sun, bright blue sky, plants just being plants and animals doing interesting things.
We sometimes use the word communion to refer to our groups divided by polity and doctrine. Those who believe as we do and associate with the same denomination as us are said to be in the same "communion." Of course, if we're in the same congregation, we gather around the same table at the same time for the Lord's supper, which is also called communion. In the PC(USA) we have an open communion. That means that we accept anyone to the table. I say "This is not my table, not a Presbyterian table; this is the Lord's table, and any who trust in the Lord are welcome." Some denominations require that all who gather at the table believe the same things. Others require certain tests of holiness (confession of sins) before allowing someone to participate in communion.
What does it mean when we say, in the Apostle's Creed, that we believe in the "communion of the saints?" Who are the saints? WE are the saints. We might think of saints as holy people and that we ourselves can't be saints because we're not holy. That's not how it works, though. All believers are justified by grace through faith. “Justified” carries the connotation that God has declared you to be holy, whether you are or not (BUT! shall we sin more abundantly that grace may abound?! by no means!). In this sense we think of all believers as holy and therefore saints.
When we commune with each other, we are present to each other, and being particularly observant of each other and enjoying each other's presence. That's a good image for the "communion of the saints." When we share in presence and fellowship, we are communing with each other and we are saints, so we create a miniature communion of the saints. Again, we PC(USA) Presbyterians have an open communion, so we're talking about any believer here, not just other Presbyterians.
But there's more than just fellowship in the celebration of the Lord's Supper. Calvin beautifully describes the role of the presence of Christ in communion. Calvin insists that Christ is not here--not in, behind, or under the bread. Christ ascended into heaven and that is where Christ is. But in Communion, according to Calvin, Christ lifts our hearts to be present with him there. Thus Presbyterians believe in a real presence of Christ in the sacrament of communion. In the same way that we've been thinking about this word, communion, in other contexts we are present to Christ and particularly observant of Christ and enjoying Christ's presence in Communion. We are "communing" with Christ. In Communion, we are communing with Christ and communing with each other. In this sense, we are united into one body, the body of Christ.
We also believe in the resurrection of the dead and the life everlasting. That means it is not only us who are communing with Christ, participating in Christ's body, but all our loved ones who have also died in Christ. There are different denominations that believe this in slightly different ways. Popular is the idea that our loved ones are already in heaven looking down on us and loving us from there. It might also be that they sleep until a moment in the future when the trumpet sounds and they awake a live and are judged by their justification to begin eternal life with Christ in the age to come. Personally, I think eternity is outside of time where we live now, and that there is no difference among the various beliefs: When we Christ lifts our hearts to heaven to enjoy his presence there, all believers of all time--past and future--are all there together. We are somehow all sucked to that one moment that is outside time. We are present to one another and particularly observant of each other and enjoy one another's presence. We are united to each other and to Christ. We, all saints in all ages, are united into the one body of Christ. To me, this is what is meant when I say I believe in the communion of the saints.
On Sunday, November 5th, we will celebrate All Saints' Day. We will serve Communion because it is the first of the month. We will also remember those who we love from our community who have died in the past year. When we say we believe in the communion of the saints, we mean the eternal fellowship of all who believe, but as we partake of Communion, I like to think that not only are our hearts lifted to heaven to enjoy Christ's presence there, but also all our loved ones are present, too, taking Communion with us. This is what we believe, and this is our hope. All blessings and comfort are ours because Christ has accomplished this communion with us and for us.