Paul calls all his fellow believers “saints” or “holy ones” and not just the notably exceptional Christian. We see an example of this in Phillipians 4, verse 21 and 22. It says: “Salute ye every saint in Christ Jesus. The brethren who are with me salute you. All the saints salute you: especially they that are of Caesar's household.” Paul also uses the term for both those who are living and for those who are dead. We find this very clearly in 2 Thess 1:9-10 and also in Jude 14-15:
2 Thess 1says:
“These (who do not acknowledge God nor heed the good news) will pay the penalty of eternal ruin, separated from the presence of the Lord and from the glory of his power, when he comes to be glorified among his holy ones and to be marveled at on that day among all who have believed, for our testimony to you was believed.”
And Jude 14-15 says:
“Enoch, of the seventh generation from Adam, prophesied also about them when he said, ‘Behold, the Lord has come with his countless holy ones to execute judgment on all and to convict everyone for all the godless deeds that they committed…”
This practice of Paul corresponds to one of the earliest creedal statements of Christian faith: The Apostles Creed: “I believe in the communion of saints.” Communion of saints refers to the bond of unity among all believers, both living and dead, who are or have been committed followers of Jesus Christ. In the eyes of God, in eternity, the distinction between His People who are ‘living’ or who are ‘dead’ is not at all important as we can see by the following Scripture verses:
Mk 9:4 “Then Elijah appeared to them along with Moses, and they were conversing with Jesus.”
Mk 12:26-27 “As for the dead being raised, have you not read in the Book of Moses, in the passage about the bush, how God told him, ‘I am the God of Abraham, (the) God of Isaac, and (the) God of Jacob’? He is not God of the dead but of the living. You are greatly misled.”
Rom 12:5 " …so we, though many, are one body in Christ and individually parts of one another."
Rom 8:38-9 "For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord."
This one body in Christ is called by the Catholic Church as The Mystical Body of Christ. This concept, as seen in the Catechism of the Catholic Church, is explained as: “The life of each of God’s children is joined in Christ and through Christ in a wonderful way to the life of all the other Christian brethren in the supernatural unity of the Mystical Body of Christ, as in a single mystical person.” (par. 1474)
Since we are “members of one another,” we can, in Christ and only in Christ, seek the prayers and help of fellow members of the Body, both here and in Heaven. Seeing as all believers as a whole make up the one body in Christ, we are all connected to each other with Christ at the head. But are those who are in heaven aware of what is happening here on earth that they could pray to God on our behalf?
What would be the point of asking for intercessory prayers if the people we are asking are not aware of us or of our prayers? Well we can find that they ARE aware of us in:
Heb 12:1 “Therefore, since we are surrounded by such a great cloud of witnesses, let us throw off everything that hinders and the sin that so easily entangles, and let us run with perseverance the race marked out for us.”
Mt 17:3 "Just then there appeared before them Moses and Elijah, talking with Jesus."
(If Jesus didn’t want any contact between saints on earth and saints in heaven, why did our Lord make a special point of appearing to Peter, James, and John on the Mount of Transfiguration in the company of Moses and Elijah, two ‘dead’ saints? (Patrick Madrid))
Rev 6:9-10 "When he opened the fifth seal, I saw under the altar the souls of those who had been slain because of the word of God and the testimony they had maintained. They called out in a loud voice, How long, Sovereign Lord, holy and true, until you judge the inhabitants of the earth and avenge our blood?"
Luke 15:10 "…There is joy in the presence of the angels of God over one sinner that repenteth."
I feel I must make clear that Jesus alone is our mediator, John Henry Cardinal Newman pointed out:
We can therefore see that asking saints to pray for us (whether they are ‘living’ or ‘dead’) is acceptable, approved by God, and availeth much. The communion of the Saints is nothing more that the recognition that saints after death (and angels) are more alive than us, aware of happenings on earth, desirous of aiding us, and able to be asked for help to assist us with their prayers of intercessions, always through Jesus, just as saints who are still ‘alive’ here on earth are able to do for us.
[Text adapted from http://www.catholicapologetics.org/ap070100.htm ]