The communion of saints is the spiritual solidarity which binds together the
faithful on earth, the
souls in
purgatory, and the saints in
heaven in the organic unity of the same
mystical body under Christ its head, and in a constant interchange of
supernatural offices. The participants in that solidarity are called saints by reason of their destination and of their partaking of the fruits of the
Redemption (
1 Corinthians 1:2 — Greek Text). The damned are thus excluded from the communion of saints. The living, even if they do not belong to the body of the
trueChurch, share in it according to the measure of their union with Christ and with the
soul of the
Church. (http://www.newadvent.org/cathen/04171a.htm)