I think many fail to recognize the book of Revelation is a beautiful representation of the Christian belief in the communion of saints. Revelation shows us the saints in heaven engaged in worship, complete with an altar, candles, vested priests, chanting the Sanctus and Alleluia, etc. The Christian liturgy on earth resembles the heavenly liturgy. For Christianity is itself a liturgical tradition.
Now Revelation 6:9...
"And when he had opened the fifth seal,
I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held: And they cried with a loud voice, saying, '
How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth?'" (Rev. 6:9-10)
The martyrs are the ones slain for their testimony and witness. (This is what the word "
martyr" means.) They are now interceding with God for those of us on Earth. They express their concern to the Lord for those on Earth. Their prayer of intercession is answered and God assures them He will trump ever though they will still be persecuted. God will clothe them in while robes as the proceeding verses tell us.
So once again early Christians built their altars over the saints to serve as a reminder of the belief in the resurrection and the communion of saints. Revelation is a beautiful express of the heavenly liturgy, which we participate in when we celebrate the Eucharist. This is why if you stepped into a church in antiquity, you would be a complete stranger and foreigner, as the faith of the early Church is completely foreign to many who claim to profess it today.