Drawing on their Jewish belief that God would raise the dead at the end of the world and witnessing to the resurrection already, in advance, of Jesus who was crucified, early Christians extended membership in their community of saints to those who had died. Teasing out the logic of this early intuition of faith, we see that they forged a certain syllogism. If living persons share in the life of God, and if the dead are likewise still clasped by the living God, then both the living and the dead are united to each other, forged into one community by the same vivifying Spirit. Paul puts this insight succinctly: 'whether we live or whether we die, we are the Lord's. For to this end Christ died and rose again, that he might be Lord of both the living and the dead' (Rom. 14:7-9). Thus the idea grew that the community of sinful yet redeemed followers of Christ not only extends across spatial boundaries to include those living in different lands at the present moment, but, as the past recedes, also stretches across time boundaries to include those living in different historical periods. The communion of saints thus expanded to include the dead according to the logic of hope in the fidelity of God.
— Elizabeth A. Johnson, Truly Our Sister: A Theology of Mary in the Communion of Saints