Q. 1. What does the "Communion of Saints" mean?
A. 1. The Communion of Saints is the spiritual union of all the Christians, living and the dead, those on earth, in Purgatory and in heaven. They share a single "mystical body", with Christ as the head, in which each member contributes to the good of all and shares in the welfare of all.
The Communion of the Saints is with Christ, through Christ, and with the Father and the Holy Spirit. "We declare to you what we have seen and heard so that you also may have fellowship with us; and truly our fellowship is with the Father and with His Son Jesus Christ." [1 Jn. 1:3]
The Communion of Saints is echoed in Colossians 1:19-20 where it states: "For in Him all the fullness was pleased to dwell, and through Him to reconcile all things for Him, making peace by the Blood of the Cross, whether those on earth or those in Heaven." [See also the Catechism of the Catholic Church # 946-948]