The Communion of Saints.
“Communion" means “fellowship.” “Saints” means “the baptised - those called to be holy - in whatever part of The Church they are” (Ephesians !, !): say, saints with a small “s” in this world, and Saints in heaven. Put shortly this article of the Creed means: - The Church is God’s Family. The Lord Jesus is our elder brother. All the baptised, in this world, in purgatory, in heaven, are brothers and sisters in Christ (Romans 12, 5).
Think, as our Lord did, of The Church as a house (St. Matthew 7, 24 to 27). It has three floors: militant, expectant, triumphant. At present you and I are on the ground floor.
We are in communion, fellowship, partnership, with the saints (the baptised, the faithful Christians) all the world over. We have the same one God and the same one faith (Ephesians 4, 5 and 6); the same sacraments; the same ten commandments. We help each other by our prayers (Acts 12,5) and by our good deeds (St. Matthew 25, 40).
On the first floor are the faithful departed, who “sleep in Jesus” (1 Thessalonians 4, 4); still in The Church; still “members of Christ” by baptism. We help them by our prayers and Masses [as St. Paul did his dead friend (2 Timothy 1, 16 to 18)]. It may be - we do not know, yet - that they too help us by their prayers.
On the top floor are The Church’s heroes and heroines, the Saints of God, members of The Church Triumphant. We are in communion and fellowship with them too. We do not pray for them: there is no need. We ask them to pray for us, that we may one day be where they are.
There are not three Churches: but one Church, one family WE DWELL IN HIM; ONE CHURCH ABOVE, BENEATH. Though I think the hymn which best explains The Communion of Saints is English Hymnal 500, Ancient and Modern 538.