I compiled a list of Catechism, Scripture and quotes from the early Church Fathers and even archaeology to assist in understanding the Communion of Saints.
You can download the source material here.
Sample:
Graffiti found in Catacombs of St. Sebastian
Who should carry the most weight—Protestant pastors protesting Catholic theology today or pastors from the early Church who have the words of the Apostles still ringing in their ears?
From earliest times Christians went to the gravesites of saints and asked them to pray for them. This picture shows graffiti from the Catacombs of St. Sebastian in Rome with inscriptions in Hebrew, Greek and Latin imploring the prayers of Peter and Paul. “Petrus and Paulus, pray for us!” “Peter and Paul, pray for victory.”
St. Cyril
"Peter and Paul, pray for us"
In 350 A.D., St. Cyril wrote a remarkable and exquisitely detailed description of the Mass, which clearly corresponds with today’s Mass. In it we find this beautiful statement on the family of God which we all belong to, and which even today we pray for in every Mass:
“Upon completion of the spiritual Sacrifice, the bloodless worship, over that propitiatory victim, we call upon God for the common peace of the Churches, for the welfare of the world, for kings, for soldiers and allies, for the sick, for the afflicted, and in summary, we all pray and offer this Sacrifice for all who are in need.
“Then we make mention also of those who have already fallen asleep: first, the patriarchs, prophets, apostles, and martyrs, that through their prayers and supplications God would receive our petition; next, we make mention also of the holy fathers and bishops who have already fallen asleep, and, to put it simply, of all among us who have already fallen asleep; for we believe that it will be of very great benefit to the souls of those for whom the petition is carried up, while this holy and most solemn Sacrifice is laid out.” (The Faith of the Early Fathers,’ Vol. 1, William A. Jurgens, [Collegeville, Minnesota: Liturgical Press, 1970] p. 363.).
The faithful in heaven and on earth are united in the Mystical Body of Christ through the Person of Jesus. This is the meaning of the Communion of Saints.