(from The Nicene and Post-Nicene Fathers: Second Series Vol. VII)

The Church includes not only those faithful who lives now on earth, but reposed as well. That is why we believe that they can pray for us to God. And experience of the Church proves it.

St. Gregory the Theologian  reminds( c. 329 – 25 January 390): "All who have lived according to God still live unto God, though they have departed this life. For this reason, God is called the God of Abraham, Isaac and Jacob, since He is the God, not of the dead, but of the living (cf. Mt. 22:32)." 

(from The Nicene and Post-Nicene Fathers: Second Series Vol. VII)