How did they differ from Protestants
How did they differ from Catholics
How did they differ from Eastern Orthodox
How did they differ from Oriental Orthodox
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If you look at the commonalities between Catholics, Eastern Orthodox, and the Oriental Orthodox churches you will find early Christianity. They met on Sunday before dawn for the liturgy and the Eucharist. Pliny, an ancient Roman governor, wrote a letter to the emperor Trajan describing the customs of the Christians of his time. In it he said:
"That they were wont, on a stated day, to meet together before it was light, and to sing a hymn to Christ, as to a god, alternately; and to oblige themselves by a sacrament [or oath], not to do anything that was ill: but that they would commit no theft, or pilfering, or adultery; that they would not break their promises, or deny what was deposited with them, when it was required back again; after which it was their custom to depart, and to meet again at a common but innocent meal..."
Notice that the hymn was sung "alternately". That is the responsive singing of the liturgy. What Pliny described as an "innocent meal" was the bread and wine of the Eucharist.
Outsiders of the time spread wild rumors about Christian worship. The Christians of the time had adopted the mother and child artistic motif to identify Jesus as a king, and they consumed his flesh and blood during the Eucharist. That led to the accusation that during their assemblies they would sacrifice a child and eat its flesh and drink its blood. Pliny points out that the meal was in fact innocent, meaning he found no evidence to suggest the rumors of cannibalism were true.
In any event, the original Apostolic Churches still meet on Sunday and still participate in the liturgy.