Catholic Encyclopedia

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 Cor 1:2). The damned are thus excluded from the communion of saints. The living, even if they do not belong to the body of the true Church, share in it according to the measure of their union with Christ and with the soul of the Church. The primitive community of Jerusalem, the disciples "devoted themselves to the apostles' teaching and fellowship, to the breaking of the bread and the prayers." Acts 2: 42 Therefore, there is a Communion in the faith. The faith of the faithful is the faith of the Church, received from the apostles. Faith is a treasure of life which is enriched by being shared. CCC 949

Communion of the sacraments. "The fruit of all the sacraments belongs to all the faithful. All the sacraments are sacred links uniting the faithful with one another and binding them to Jesus Christ, and above all Baptism, the gate by which we enter into the Church. The communion of saints must be understood as the communion of the sacraments. . . . The name 'communion' can be applied to all of them, for they unite us to God. . . . But this name is better suited to the Eucharist than to any other, because it is primarily the Eucharist that brings this communion about." ^[Roman Catechism I,10,24] CCC 950